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TUNNELING: 

A PRACTICAL TREATISE 

BY 

CHARLES PRELINI, C. E. 

WITH ADDITIONS BY 

CHARLES S. HILL, C. E, 

ASSOCIATE EDITOR "ENGINEERING NEWS" 

Z50 DIAGRAMS AND ILLUSTRATIONS 
SECOND EDITION, REVISED. 




NEW YORK: 

D. VAN NOSTRAND COMPANY 

23 Murray and 27 Warren Sts. 
1902 



8 



i 



^f^,f3 



TKF LfBRARY OF 
CONGRESS, 

Two Coptea Received 

iUL. 24 1902 

nOPVRIQHT ENTRY 

CLASS WxXa No. 

3 t 2^ ^ -| 
COPY 8. I 



Copyright, 1901, 1902. 
D. VAN NOSTRAND COMPANY. 




y 



/ 







• • •-• 



TVPOGRAPHY BY 

C. J. Peters & Son, 

BOSTON, MASS. 
U.S.A. 



PEEFACE 



In his work at Manhattan College the writer found himself 
confronted by the fact that there were but two books on Tun- 
neling in the English language, neither of which he could rec- 
ommend as text books for his pupils. Drinker's tunneling is 
a splendid reference book, and may be consulted with advan- 
tage by any engineer, but it is too voluminous and expensive 
to be suitable for the beginner. Simms's Practical Tunnel- 
ing is a magnificent exposition of the English method of 
tunneling, but it is too old for anyone who looks for the most 
modern methods of tunneling, as the art has progressed greatly 
since Mr. Simms's death. The additions introduced by Mr. 
D. K. Clarke, although they convey an excellent idea of the 
manner of excavating long tunnels like the Mont Cenis and 
St. Gothard, fail in what may be called real practical value, 
viz., to explain to engineers and contractors the various meth- 
ods of driving tunnels of ordinary dimensions through different 
soils. 

Having thus felt the want of a book of convenient size and 
moderate price, the author began to enlarge the notes of his 
lectures for publication. The general purpose of the book 
which has resulted is to explain all the operations that are re- 
quired in tunneling, and then illustrate by suitable examples 
the actual application of these methods in practice. Formulas 
and difficult calculations have been avoided, the book being 
simply descriptive, and the text well illustrated, so that it can 
be easily understood by students and others unfamiliar with 
tunneling work. Tnis work of preparation has been very diffi- 
cult to the writer owing to the fact that, being a foreigner, the 



iv PREFACE 

language was not fully mastered, and, having been but a few 
years in this country, he was not familiar with what had been 
accomplished here in previous years. The latter fault was 
remedied as far as possible by a careful consultation of the 
Transactions of the American Society of Civil Engineers, the 
volumes of Engineering JVews, and of other periodicals, with 
all of which the writer made very free use. The writer has 
received much assistance from his friends in the preparation of 
the manuscript, and takes this opportunity to thank them for 
their trouble and encouragement. It is his wish, however, to 
give special thanks to Mr. Charles S. Hill, Associate Editor 
Engineering News, whose suggestions and criticisms led to many 
changes and additions, and extended the scope of the book. 

Chakles Pkblini. 
February, 1901. 



OOISTTEISTTS 



PAGE 

INTRODUCTION — The Historical Development op Tunnel Build- 
ing i^ 

CHAPTER 

I. Preliminary Considerations ; Choice Between a Tunnel 
AND AN Open Cut ; Method and Purpose of Geological 

Surveys • » • ^ 

II. Methods of Determining the Center Line and Forms and 

Dimensions of Cross-Section ^ 

III. Excavating Machines and Rock Drills; Explosives and 

Blasting 1^ 

IV. General Methods of Excavation ; Shafts ; Classification 

OF Tunnels 2^ 

V. Methods of Timbering or Strutting Tunnels 4a 

VI. Methods of Hauling in Tunnels 5& 

VII. Types of Centers and Molds Employed in Constructing 

Tunnel Linings of Masonry 62^ 

VIII. Methods of Lining Tunnels 6& 

IX. Tunnels through Hard Rock ; General Discussion ; Exca- 
vation BY Drifts ; Mont Cenis Tunnel 79^ 

X. Tunnels through Hard Rock {continued) ; The Simplon 

Tunnel 94 

XI. Tunnels through Hard Rock (continued) ; Excavation by 

Headings; St. Gothard Tunnel; Busk Tunnel 114 

XII. Representative Mechanical Installations for Tunnel 

Work 124 

XIII. Excavating Tunnels through Soft Ground ; General Dis- 

cussion ; The Belgian Method 13^ 

XIV. The German Method of Excavating Tunnels through 

Soft Grouxd ; Baltimore Belt-Line Tunnel . . . . . 145^ 
XV. The Full-Section Method op Tunneling ; English Method, 

Austrian Method 15ft 

V 



VI CONTENTS 

CHAPTER PAOB 

XVI. Special Treacherous Ground Method ; Italian Method ; 

Quicksand Tunneling ; Pilot Method ....... 167 

XVII. Open -Cut Tunneling Methods; Tunnels under City 

Streets ; Boston Subway and New York Eapid Transit, 180 
XVIII. Submarine Tunneling ; General Discussion ; The Severn 

Tunnel 201 

XIX. Submarine Tunneling (continued) ; The East River Gas 

Tunnel; The Van Buren Street Tunnel, Chicago . . 208 
XX. Submarine Tunneling {continued) ; The Milwaukee Water- 
Works Tunnel 230 

XXI. Submarine Tunneling (continued) ; The Shield System . . 242 
XXII. Accidents and Repairs in Tunnels during and after Con- 
struction 266 

XXIII. Relining Timber-Lined Tunnels with Masonry .... 280 

XXIV. The Ventilation and Lighting of Tunnels during Con- 

struction , 290 

XXV. The Cost of Tunnel Excavation, and the Time Required 

for the Work 300 

Index SOP 



IITTRODUOTIOlNr 



THE HISTORICAL DEVELOPMENT OF TUNNEL 

BUILDING. 

A TUNNEL, defined as an engineering structure, is an artificial 
gallery, passage, or roadway beneath the ground, under the bed 
of a stream, or through a hill or mountain. The art of tunnel- 
ing has been known to man since very ancient times. A The- 
ban king on ascending the throne began at once to drive the 
long, narrow passage or tunnel leading to the inner chamber or 
sepulcher of the rock-cut tomb which was to form his final 
resting-place. Some of these rock-cut galleries of the ancient 
Egyptian kings were over 750 ft. long. Similar rock-cut tun- 
neling work was performed by the Nubians and Indians in 
building their temples, by the Aztecs in America, and in fact 
by most of the ancient civilized peoples. 

The first built-up tunnels of which there are any existing 
records were those constructed by the Assyrians. The vaulted 
drain or passage under the southeast palace of Nimrud, built by 
Shalmaneser II. (860-824 B.C.), is in all essentials a true soft- 
ground tunnel, with a masonry lining. A much better exam- 
ple, however, is the tunnel under the Euphrates River, which 
may quite accurately be claimed as the first submarine tunnel 
of which there exists any record. It was, however, built under 
the dry bed of the river, the waters of which were temporarily 
diverted, and then turned back into their normal channel after 
the tunnel work was completed, thus making it a true sub- 
marine tunnel only when finished. The Euphrates River tun- 
nel was built through soft ground, and was lined with brick 



X INTKODUCTION 

masonry, having interior dimensions of 12 ft. in width and 15 
ft. in height. 

Only hand labor was employed by these ancient peoples in 
their tunnel work. In soft ground the tools used were the 
pick and shovels, or scoops. For rock work they possessed a 
greater range of appliances. Research has shown that among 
the Egyptians, by whom the art of quarrying was highly de- 
veloped, use was made of tube drills and saws provided with 
cutting edges of corundum or other hard, gritty material. The 
usual tools for rock work were, however, the hammer, the chisel, 
and wedges ; and the excellence and magnitude of the works 
accomplished by these limited appliances attest the unlimited 
time and labor which must have been available for their ac- 
complishment. 

The Romans should doubtless rank as the greatest tunnel 
builders of antiquity, in the number, magnitude, and useful 
character of their works, and in the improvements which they 
devised in the methods of tunnel building. They introduced 
hre as an agent for hastening the breaking down of the rock, 
and also developed the familiar principle of prosecuting the 
work at several points at once by means of shafts. In their 
use of fire the Romans simply took practical advantage of the 
familiar fact that when a heated rock is suddenly cooled it 
cracks and breaks so that its excavation becomes comparatively 
easy. Their method of operation was simply to build large 
fires hi front of the rock to be broken down, and when it had 
reached a high temperature to cool it suddenly by throwing 
water upon the hot surface. The Romans were also aware 
that vinegar affected calcareous rock, and in excavating tunnels 
through this material it was a common practice with them to 
substitute vinegar for water as the cooling agent, and thus to 
attack the rock both chemically and mechanicall3r. It is hardly 
necessary to say that this method of excavation was very severe 
on the workmen because of the heat and foul gases generated. 
This was, however, a matter of small concern to the builders. 



INTRODUCTION XI 

since the work was usually performed by slaves and prisoners 
of war, wlio perislied by thousands. To be sentenced to labor 
on Roman tunnel works was thus one of the severest penalties 
to which a slave or prisoner could be condemned. They were 
places of suffering and death as are to-day the Spanish mercury 
mines. 

Besides their use of fire as an excavating agent, the Romans 
possessed a very perfect knowledge of the use of vertical shafts 
in order to prosecute the excavation at several different points 
simultaneously. Pliny is authority* for the statement that in 
the excavation of the tunnel for the drainage of Lake Fucino 
forty shafts and a number of inclined galleries were sunk along 
its length of 3^ miles, some of the shafts being 400 ft. in 
depth. The spoil was hoisted out of these shafts in copper 
pails of about ten gallons' capacity by windlasses. 

The Roman tunnels were designed for public utility. Among 
those which are most notable in this respect, as well as for 
being fine examples of tunnel work, may be mentioned the nu- 
merous conduits driven through the calcareous rock between 
Subiaco and Tivoli to carry to Rome the pure water from the 
mountains above Subiaco. This work was done under the 
Consul Marcius. The longest of the Roman tunnels is the one 
built to drain Lake Fucino, as mentioned above. This tunnel 
was designed to have a section of 6 ft. x 10 ft. ; but its actual 
dimensions are not uniform. It was driven through calcareous 
rock, and it is stated that 30,000 men were employed for eleven 
years in its construction. The tunnels which have been men- 
tioned, being designed for conduits, were of small section ; but 
the Romans also built tunnels of larger sections at numerous 
points along their magnificent roads. One of the most notable 
of these is that which gives the road between Naples and Poz- 
zuoli passage through the Posilipo hills. It is excavated 
through volcanic tufa, and is about 3000 ft. long and 25 ft. 
wide, with a section of the form of a pointed arch. In order 

* "Tunneling," Encly. Brit., 1889, vol. xxiii., p. 623. 



XU INTRODUCTION 

to facilitate the illumination of this tunnel, its floor and roof 
were made gradually converging from the ends toward the 
middle ; at the entrances the section was 75 ft. high, while at 
the center it was only 22 ft. high. This double funnel-like 
construction caused the rays of light entering the tunnel to 
concentrate as they approached the center, and thus to improve 
the natural illumination. The tunnel is on a grade. It was 
probably excavated during the time of Augustus, although 
some authorities place its construction at an earlier date. 

During the Middle Ages the art of tunnel building was 
practiced for military purposes, but seldom for the public need 
and comfort. Mention is made of the fact that in 1450 Anne 
of Lusignan commenced the construction of a road tunnel 
under the Col di Tenda in the Piedmontese Alps to afford 
better communication between Nice and Genoa ; but on account 
of its many difficulties the work was never completed, although 
it was several times abandoned and resumed. For the most 
part, therefore, the tunnel work of the Middle Ages was in- 
tended for the purposes and necessities of war. Ever}" castle 
had its private underground passage from the central tower or 
keep to some distant concealed place to permit the escape of 
the family and its retainers in case of the victory of the enemy, 
and, during the defense, to allow of sorties and the entrance 
of supplies. 

The tunnel builders of the Middle Ages added little to the 
knowledge of their art. Indeed, until the 17th century and 
the invention of gunpowder no practical improvement was 
made in the tunneling methods of the Romans. Engravings 
of mining operations in that century show that underground 
excavation was accomplished by the pick or the hammer and 
chisel, and that wood fires were lighted at the. ends of the 
headings to split and soften the rocks in advance. Although 
gunpowder had been previously employed in mining, the first 
important use of it in tunnel work was at Malpas, France, 
in 1679-81, in the tunnel for the Languedoc Canal. This 



INTRODUCTION XIU 

tunnel was 510 ft. long, 22 ft. wide, and 29 ft. high, and was 
excavated through tufa. It was left unlined for seven years, 
and then was lined with masonry. 

With the advent of gun230wder and canal building the first 
strong impetus was given to tunnel building, in its modern 
sense, as a commercial and public utilitarian construction, since 
the days of the Roman Empire. Canal tunnels of notable 
size were excavated in France and England during the last 
half of the 17th century. These were all rock or hard-ground 
tunnels. Indeed, previous to 1800 the soft-ground tunnel was 
beyond the courage of engineer except in sections of such 
small size that the work better deserves to be called a drift or 
heading than a tunnel. In 1803, however, a tunnel 24 ft. 
wide was excavated through soft soil for the St. Quentin Canal 
in France. Timbering or strutting was employed to support 
the walls and roof of the excavation as fast as the earth was 
removed, and the masonry lining was built closely following it. 
From the experience gained in this tunnel were developed the 
various systems of soft-ground subterrannean tunneling since 
employed. 

It was by the development of the steam railway, however, 
that the art of tunneling was to be brought into its present 
prominence. In 1820-26 two tunnels were built on the Liver- 
pool & Manchester Ry. in England. This was the beginning 
of the rapid development which has made the tunnel one of 
the most familiar of engineering structures. The first railway 
tunnel in the United States was built on the Alleghany & 
Portage R.R. in Pennsylvania in 1831-33 ; and the first canal 
tunnel had been completed about 13 years previously (1818-21 ) 
by the Schuylkill Navigation Co., near Auburn, Pa. It would 
be interesting and instructive in many respects to follow the 
rise and progress of tunnel construction in detail since the con- 
struction of these earlier examples, but all that may be said 
here is that it was identical with that of the railway. 

The art of tunneling entered its last and greatest phase 



XIV INTKODUCTION 

with the construction of the Mont Cenis tunnel in Europe and 
the Hoosac tunnel in America, which works established the 
utility of machine rock-drills and high explosives. The Mont 
Cenis tunnel was built to facilitate railway communication 
between Italy and France, or more properly between Pied- 
mont and Savoy, the two parts of the kingdom of Victor 
Emmanuel IL, separated by the Alps. It is 7.6 miles long, 
and passes under the Col di Frejus near Mont Cenis. Som- 
meiller, Grattoni, and Grandis were the engineers of this great 
undertaking, which was begun in 1857, and finished in 1872. 
It was from the close study of the various difficulties, the great 
length of the tunnel, and the desire of the engineers to finish 
it quickly, that all the different improvements were developed 
which marked this work as a notable step in the advance of 
the art of tunneling. Thus the first power-drill ever used in 
tunnel work was devised by Sommeiller. In addition, com- 
pressed air as a motive power for drills, aspirators to suck the 
foul air from the excavation, air compressors, turbines, etc., 
found at Mont Cenis their first application to tunnel construc- 
tion. This important role played by the Mont Cenis tunnel 
in Europe in introducing modern methods had its counterpart 
in America in the Hoosac tunnel completed in 1875. In this 
work there were used for the first time in America power rock- 
drills, air compressors, nitro-glycerine, electricity for firing 
blasts, etc. 

There remains now to be noted only the final development 
in the art of soft-ground submarine tunneling, namely, the use 
of the shield and metal lining. The shield was invented and 
first used by Sir Isambard Brunei in excavating the tunnel 
under the River Thames at London, which was begun in 1825, 
and finished in 1841. In 1869 Peter William Barlow used an 
iron lining in connection with a shield in driving the second 
tunnel under the Thames at London. From these inventions 
has grown up one of the most notable systems of tunneling 
now practiced, which is commonly known as the shield system. 



INTRODUCTION XV 

In closing this brief review of tlie development of modern 
methods of tunneling, to the presentation of which the re- 
mainder of this book is devoted, mention should be made of 
a form of motive power which promises many opportunities for 
development in tunnel construction. Electricity has long been 
employed for blasting and illuminating purposes in tunr.el 
work. It remains to be extended to other uses. For hauling 
and for operating certain classes of hoisting and excavating 
machinery it is one of the most convenient forms of power 
available to the engineer. Its successful application to rock- 
drills is another promising field. For operating ventilating 
fans it promises unusual usefulness. 



TUNNELING 



CHAPTER I 

PRELIMINARY CONSIDERATIONS. CHOICE BE- 

TWEEN A TUNNEL AND OPEN CUT. 

GEOLOGICAL SURVEYS 



CHOICE BETWEEN A TUNNEL AND AN OPEN CUT 

When a railway line is to be carried across a range of 
mountains or hills, the first question which arises is whether 
it is better to construct a tunnel or to make such a detour as 
will enable the obstruction to be passed with ordinary surface 
construction. The answer to this question depends upon the 
comparative cost of construction and maintenance, and upon 
the relative commercial and structural advantages and disad- 
vantages of the two methods. In favor of the open road there 
are its smaller cost and the decreased time required in its con- 
struction. These mean that less capital will be required, and 
that the road will sooner be able to earn something for its 
builders. Against the open road there are : its greater length 
and consequently its heavier running expenses ; the greater 
amount of rolling-stock required to operate it ; the heavy ex- 
pense of maintaining a mountain road ; and the necessity of 
employing larger locomotives, with the increased expenses which 
they entail. In favor of the tunnel there are : the shortening 
of the road, with the consequent decrease in the operating 
expenses and amount of rolling-stock required ; the smaller cost 

1 



2 TUNNELING 

■of maintenance, owing to the pi-otection of the track from snow 
.and rain and other natural influences causing deterioration ; 
and the decreased cost of hauling due to the lighter grades. 
Against the tunnel, there are its enormous cost as compared 
with an open road and the great length of time required to 
construct it. 

To determine in any particular case whether a tunnel or an 
•open road is best, requires a careful integration of all the factors 
mentioned. It may be asserted in a general way, however, that 
the enormous advance made in the art of tunnel building has 
done much to lessen the strength of the principal objections to 
tunnels, namely, their great cost and the length of time required 
for their construction. Where the choice lies between a tunnel 
or a long detour with heavy grades it is sooner or later almost 
always decided in favor of a tunnel. When, however, the con- 
ditions are such that the choice lies between a tunnel or a 
heavy open cut with the same grades the problem of deciding 
l)etween the two solutions is a more difficult one. 

It is generally assumed that when the cut required will have 
a vertical depth exceeding 60 ft. it is less expensive to build 
a tunnel unless the excavated material is needed for a nearby 
■embankment or fill. This rule is not absolute, but varies 
according to local conditions. For instance, in materials of 
rigid and unyielding character, such as rock, the practical limit 
to the depth of a cut goes far beyond that point at which a 
tunnel would be more economical according to the above rule. 
In soils of a yielding character, on the other hand, the very 
flat slope required for stability adds greatly to the cost of 
making a cut. 

It may be noted in closing that the same rule may be em- 
ployed in determining the location of the ends of the tunnel, 
for assuming that it is more convenient to excavate a tun- 
nel than an open cut when the depth exceeds 60 ft., then 
the open cut approaches sliould extend into the mountain- or 
hill-sides only to tlie points where the surface is 60 ft. above 



CHOICE BETWEEN A TUNNEL AND AN OPEN CUT 3 

grade, and there the tunnel should begin. If, therefore, we 
draw on the longitudinal profile of the tunnel a line parallel to 
the plane of the tracks, and 60 ft. above it, this line will cut 
the surface at the points where the open-cut approaches should 
cease and the tunnel begin. This is a rule-of-thumb determi- 
nation at the best, and requires judgment in its use. Should 
the ground surface, for example, rise only a few feet above the 
60 ft. line for any distance, it is obviously better to continue 
the open cut than to tunnel. 

THE METHOD AND PURPOSE OF GEOLOGICAL SURVEYS 

When it has been decided to build a tunnel, the first duty 
of the engineer is to make an accurate geological survey of 
the locality. From this survey the material penetrated, the 
form of section and kind of strutting to be used, the best form 
of lining to be adopted, the cost of excavation, and various 
other facts, are to be deduced. In small tunnels the geological 
knowledge of the engineer should enable him to construct a 
geological map of the locality, or this knowledge may be had 
in many cases by consulting the geological maps issued by the 
State or general government surveys. When, however, the 
tunnel is to be of great length, it may be necessary to call in 
the assistance of a professional geologist in order to reconstruct 
accurately the interior of the mountain and thereby to ascer- 
tain beforehand the different strata and materials to be 
excavated, thus obtaining the data for calculating both the 
time and cost of excavating the tunnel. 

The geological survey should enable the engineer to deter- 
mine, (1) the character of the material and its force of cohe- 
sion, (2) the inclination of the different strata, and (3) the 
presence of water. 

Character of Material. — The character of the material through 
which the proposed tunnel will penetrate is best ascertained 
by means of diamond rock-drills. Tliese machines bore an 



4 TUNNELING 

annular hole, and take away a core for the whole depth of the 
boring, thus giving a perfect geological section showing the 
character, succession, and exact thickness of the strata. By 
making such borings at different points along the center line 
of the projected tunnel, and comparing the relative sequence 
and thickness of the different strata shown by the cores, the 
geological formation of the mountain may be determined quite 
exactly. Where it is difficult or impracticable to make dia- 
mond drill borings on account of the depth of the mountain 
above the tunnel, or because of its inaccessibility, the engineer 
must resort to other methods of observation. 

The present forms of mountains or hills are due to 
weathering, or the action of the destructive atmospheric influ- 
ences upon the original material. From the manner in which 
the mountain or hill has resisted weathering, therefore, may be 
deduced in a general way both the nature and consistency of 
the materials of which it is composed. Thus we shall gener- 
ally find mountains or hills of rounded outlines to consist 
of soft rocks or loose soils, while under very steep and crested 
mountains hard rock usually exists. To the general knowl- 
edge of the nature of its interior thus afforded by the ex- 
terior form of the mountain, the engineer must add such 
information as the surface outcroppings and other local evi- 
dences permit. 

For the purposes of the tunnel builder we may first classify 
all materials as either, (1) hard rock, (2) soft rock, or (3) 
soft soil. 

Hard rocks are those having sufficient cohesion to stand 
vertically when cut to any depth. Many of the primary rocks, 
like granite, gneiss, feldspar, and basalt, belong to this class, 
but others of the same group are affected by the atmosphere, 
moisture, and frost, which gradually disintegrate them. They 
are also often found interspersed with pyrites, whose well- 
known tendency to disintegrate upon exposure to air intro- 
duces another destructive agency. For these reasons we may 



CHOICE BETWEEN A TUNNEL AND AN OPEN CUT 5 

divide hard rocks into two sub-classes ; viz., hard rocks un- 
affected by tlid atmosphere, and those affected by it. This 
distinction is chiefly important in tunneling as determining 
whether or not a lining will be required. 

Soft rocks, as the term implies, are those in whicli the force 
of cohesion is less than in hard rocks, and which in consequence 
offer less resistance to attacks tending to break down their 
original structure. They are always affected by the atmosphere. 
Sandstones, laminated clay shales, mica-schists, and all schistose 
stones, chalk and some volcanic rocks, can be classified in this 
group. Soft rocks require to be supported by timbering during 
excavation, and need to be protected by a strong lining to 
exclude the air, and to support the vertical pressures, and 
prevent the fall of fragments. 

Soft soils are composed of detrital materials, having so little 
cohesion that they may be excavated without the use of 
explosives. Tunnels excavated through these soils must be 
strongly timbered during excavation to support the verti- 
cal pressure and prevent caving ; and they also always require 
a strong lining. Gravel, sand, shale, clay, quicksand, and peat 
are the soft soils generally encountered in the excavation of 
tunnels. Gravels and dry sand are the strongest and firmest ; 
shales are very firm, but they possess the great defect of being 
liable to swell in the presence of water or merely by exposure 
to the air, to such an extent that they have been known to 
crush the timbering built to support them. Quicksand and 
peat are proverbially treacherous materials. Clays are some- 
times firm and tenacious, but when laminated and in the 
presence of water are among the most treacherous soils. 
Laminated clays may be described as ordinary clays altered 
by chemical and mechanical agencies, and several modifications 
of the same structure are often found in the same locality. 
They are composed of laminae of lenticular form separated by 
smooth surfaces and easily detached from each other. Lami- 
nated clays generally have a dark color, red, ocher or greenish 



6 TUNNELING 

blue, and are very often found alternating with strata of 
stiatites or calcareous material. For purposes of construction 
they have been divided into three varieties. 

Laminated clays of the first variety are those which alter- 
nate with calcareous strata and are not so greatly altered as 
to lose their original stratification. Laminated clays of the 
second variety are those in which the calcareous strata are 
broken and reduced to small pieces, but in which the former 
structure is not completely destroyed ; the clay is not reduced 
to a humid state. Laminated clays of the third variety 
are those in which the clay by the force of continued disturb- 
ance, and in the presence of water, has become plastic. 
Laminated clays are very treacherous soils ; quicksand and 
peat may be classed, as regards their treacherous nature, 
among the laminated clays of the third variety. 

Inclination of Strata Knowing the inclination of the 

strata, or the angle which they make with the horizon, it is 
easy to determine where they intersect the vertical plane of the 
tunnel passing through the center line, thus giving to a certain 
extent a knowledge of the different strata which will be met 
in the excavatioUo On the inclination of the strata depend : 
(1) The cost of the excavation ; the blasting, for instance, will 
be more efficient if the rocks are attacked perpendicular to the 
stratification ; (2) The character of the timbering or strut- 
ting ; the tendency of the rock to fall is greater if the strata 
are horizontal than if they are vertical ; (3) The character and 
thickness of the lining ; horizontal strata are in the weakest 
position to resist the vertical pressure from the load above 
when deprived of the supporting rock below, while vertical 
strata, when penetrated, act as a sort of arch to support the 
pressure of the load above. The foregoing remarks apply 
only to hard or soft rock materials. 

In detrital formations the inclination of the strata is an 
important consideration, because of the unsymmetrical pres- 
sures developed. In excavating a tunnel through soft soil 



CHOICE BbyrWEEX A TUNNEL x\ND AN OPEN CUT T 

whose strata are inclined at 30° to the horizon, for instance, 
the tunnel will cut these strata at an angle of 30°. By the 
excavation the natural equilibrium of the soil is disturbed, 
and Avdiile the earth tends to fall and settle on both sides 
at an angle depending upon the friction and cohesion of the- 
material, this angle will be much greater on one side than on 
the other because of the inclination of the strata; and hence 
the prism of falling earth on one side is greater than on the- 
other, and consequently the pressures are different, or in 
other words, they are unsymmetrical. These unsym metrical 
pressures are usually easily taken care of as far as the lining 
is concerned, but they may cause serious cave-ins and badly^ 
distort the strutting. Caving-in during excavation may be 
prevented by cutting the materials according to their natural 
slope ; but the distortion of the strutting is a more serious 
problem to handle, and one which oftentimes requires the 
utmost vigilance and care to prevent serious trouble. 

Presence of Water. — An idea of the likelihood of finding- 
water in the tunnel may be obtained by studying the hydro- 
graphic basin of the locality. From it the source and direction 
of the springs, creeks, ravines, etc., can be traced, and from 
the geological map it can be seen where the strata bearing- 
these waters meet the center line. Not only ought the surface 
water to be attentively studied, but underground springs, which 
are frequently encountered in the excavation of tunnels, re- 
quire careful attention. Both the surface and underground 
waters follow the pervious strata, and are diverted by im- 
pervious strata. Rocks generally may be classed as im- 
pervious ; but they contain crevices and faults, which often 
allow water to pass tlirough them ; and it is, therefore, not 
uncommon to encounter large quantities of water in excavating 
tunnels through rock. As a rule, water will be found under 
high mountains, winch comes from the melted ice and snow 
percolating thiough the rock crevices. 

Some detrital soils, like gravel and sand, are pervious, and 



8 TUNNELING 

others, like clay and shale, are impervious. Detrital soils 
lying above clay are almost certain to carry water just above 
the clay stratum. In tunnel work, therefore, when the exca- 
vation keeps well within the clay stratum, little trouble is 
likely to be had from water ; should, however, the excavation 
cut the clay surface and enter the pervious material above, 
water is quite certain to be encountered. The quantity of 
water encountered in any case depends upon the presence of 
high mountains near by, and upon other circumstances which 
will attract the attention of the engineer. 

A knowledge of the pressure of the water is desirable. 
This may be obtained by observing closely its source and the 
character of the strata through which it passes. Water 
coming to the excavation through rock crevices will lose 
little of its pressure by friction, while that which has passed 
some distance through sand will have lost a great deal of its 
pressure by friction. Water bearing sand, and, in fact, any 
water bearing detrital material, has its fluidity increased by 
water pressure ; and when this reaches the point where flow 
results, trouble ensues. The streams of water met in the 
construction of the St. Gothard tunnel had sufficient pressure 
to carry away timber and materials. 



DETERMINING THE CENTER LINE 



CHAPTER II. 

METHODS OF DETERMINING THE CENTER 

LINE AND FORMS AND DIMENSIONS OF 

CROSS-SECTION. 



DETERMINING THE CENTER LINE. 

Tunnels may be either curvilinear or rectilinear, but the 
latter form is the more common. In either case the first task 
of the engineer, after the ends of the tunnel have been definitely 
fixed, is to locate the center Ime exactly. This is done on the 
surface of the ground; and its purpose is to find the exact 
length of the tunnel, and to furnish a reference line by which 
the excavation is directed. 

Rectilinear Tunnels. — In short tunnels the center line may be 
accurately enough located for all practical purposes by means 
of a common theodolite. The work is performed on a calm, 
clear day, so as to have the instrument and observations sub- 
jected to as little atmospheric disturbance as possible. Wooden 
stakes are employed to mark the various located points of the 
center line temporarily. The observations are usually repeated 
once at least to check the errors, and the stakes are altered as 
the corrections dictate ; and after the line is finally decided to 
be correctly fixed, they are replaced by permanent monu- 
ments of stone accurately marked. The method of checking the 
observations is described by Mr. W. D. Haskoll * as follows : 

" Let the theodolite be carefully set up over one of the stakes, with the 
nail driven into it, selecting? one that will command the best position so as to 
ranj^e backwards and forwards over the whole length of line, and also obtain a 
view of the two distant points that range with the center line ; this being done, 

* " Practical Tunneling," by F. W. Simms. 



10 TUNNELING 

let the centers of every stake ... be carefully verified. If this be carefully 
done, and the centers be found correct, and thoroughly in one visual line as 
seen through the telescope, there vs^ill be no fear but that a perfectly straight 
line has been obtained. 

The center line which has thus been located on the ground 
surface has to be transposed to the inside of the tunnel to 
direct the excavation. To do this let A and B be the entrances 
and a and b be the two distinct fixed points which have been 
ranged in with the center line located on the ground surface 
over the hill AfB, Fig. 1. The instrument is set up at F", 
any point on the line A a produced, and a bearing secured by 
observation on the center line marked on the surface. This 
bearing is then carried into the tunnel by plunging the tele- 
scope, and setting pegs in the roof of the heading. Lamps 



B"' A 

Fig. 1. — Diagram Showing Manner of Lining in Rectilinear Tunnels. 

hung from these pegs furnish the necessary sighting points. 
This same operation is repeated on the opposite side of the 
hill to direct the excavation from that end of the tunnel. 
These operations serve to locate only the first few points inside 
the tunnel. As the excavation penetrates farther into the hill, 
it becomes impossible to continue to locate the line from the 
outside point, and the line has to be run from the points 
marked on the roof of the heading. Great accuracy is required 
in all these observations, since a very small error at the begin- 
ning becomes greater and greater as the excavation advances. 

In very long tunnels excavated under high mountains more 
elaborate methods have to be adopted for locating the center 
line. The theodolites employed must be of large size ; in ran- 
ging the center line of the St. Gothard tunnel, the theodolite 
used had an object glass eight inches in diameter.* Instead of 

* See also Simplon Tunnel, Chapter IX. 



DETEKMLSIXG THE CENTEK LINE 



11 



the ordinary mounting a masonry pedestal witli a perfectly 
level top is employed to support the instrument during the 
observations. The location is made by means of triangulation. 
The various operations must be performed with the greatest 
accuracy, and repeated several times in sucii a way as to reduce 
the errors to a minimum, since the final meeting of the head- 
ings depends upon their elimination. 

The St. Gothard tunnel furnishes perhaps the best illus- 
tration of careful work in locating the center line of long recti- 
linear tunnels of any tunnel ever built. The length of this 
tunnel is 9.25 miles, and the height of the mountain above it 
is very great. The center line was located by triangulation by 



V.Boreh 



m.Stabbielh 




mfibbla 
Fig. 2. — Triangulation System for Establishing the Center Line of the St. Gothard Tunnel. 

two different astronomers using different sets of triangles, and 
working at different times. The set or system of triangles used 
by Dr. Koppe, one of the observers, is shown by Fig. 2 ; it con- 
sists of very large and quite small triangles combined, the 
latter being required because the entrances both at Airolo and 
Goeschenen were so low as to permit only of a short sight 
being taken. The apices of the triangles were located by means 
of the contour maps of the Swiss Alpine Club. Each angle 
was read ten times, the instrument was collimated four times 
for each reading, and was afterwards turned off 5° or 10° to 
avoid errors of graduation. The average of the errors in read- 
ing was about one second of arc. The triangulation was coinpen- 



12 



TUNNELING 



Wire 



sated according to the method of least squares. The probable 
error in the fixed direction was calculated to be 0.8'' of arc at 
Goeschenen and 0.7'' of arc at Airolo. From this it was 
assumed that the probable deviation from the true center would 
be about two inches at the middle of the tunnel, but when the 
headings finally met this deviation was found to reach eleven 
inches. 

Comparatively few tunnels are driven by working from the 
entrances alone, the excavation being usually prosecuted at 
several points at once by means of shafts. In these cases, in 

order to direct the excavation cor- 
rectly, it is necessary to fix the 
center line on the bottom of the 
shaft. This is accom.plished in 
two ways, — one being employed 
when the shaft is located directly 
over the center line, and the other 
when the shaft is located to one 
side of the center line. 

When the shaft is located on 
the center line two small pillars 
are placed on opposite edges of 
the shaft and collimating with the 
center line as shown by Fig. 3. 
On these two pillars the points 
corresponding to the center line are correctly marked, and con- 
nected by a wire stretched between them. To this wire two 
plumb bobs are fastened as far apart as .possible. These plumb 
bobs mark two points on the center line at the bottom of the 
shaft, and from them the line is extended into the headings as 
the work advances. Compass readings are employed to check 
the transit lines ranged on the plumb bobs. Where there are 
rocks containing iron ore a miner's transit should be employed 
for making the compass reading. 

When the shaft is placed at one side of the tunnel the 




Fig. 3.— Method of Transferring the 
Center Line down Center Shafts. 



DETERMINING THE CENTER LINE 



13 



Fig. 4 



m 

i 



■Y 

|W 



Method of Transferring the Center 
Line down Side Shafts. 



pillars or bench marks are placed normal to the center line on 

the edges of the shaft as shown by Fig. 4. Between the points 

A and B a wire is stretched, and from it two plumb bobs are 

suspended, as described in the 

preceding case ; these plumb 

bobs establish a vertical plane 

normal to the axis of the tun- 
nel. The excavation of the 

side tunnel is carried alono- the 

line ^TF until it intersects the 

line of the main tunnel, whose 

center line is determined by center' -o ~I//7^ 

measurinor off undero-round a 

distance equal to the distance 

BO on the surface. By setting 

the instrument over the under-ground point 0, and turning off 

a right angle from the line BO^ the center line of the tunnel is 

extended into the headings. 

Curvilinear Tunnels. — There are various methods of locating 

the center line of curvilinear tunnels, but the method of tangent 
offsets is the one most commonly employed. It 
consists in finding the length of an ordinate 
DC^ Fig. 5^ perpendicular to the tangent JLX, 
at a point D taken at a known distance AD 
= d from tlie point of tangent A, being the 
center of the arc AB and OA being the radius. 
Pig. 5. — Diagram From draw OZ parallel to the tangent AX^ 
Showing Method ^^^ producc the perpendicular DC until it in- 

of Determining ^ r l 

Tangent Offsets tcrsccts tlic line OZ at E. Joiu and C, 

for Arcs of 90*^ 

or Less. From the right^angle triangle OCE^ 

OE = AD = d 

CO = r 

EC = Vr2 - (P . . 

ED = OA = r 

DC = r - EC = ij . 




(1) 
(2) 



14 



TUNNELING 



Substituting these values in equations (1) and (2) we 
have, y = r — Vr' — (T. 

When the arc AB is greater than a quadrant, as in Fig. 6, 
the projection AF of the arc becomes equal to its radius, and 

for any value of d between this pro- 
jection and that of the chord AK 
there are thus two values of ^, viz., 




Vi 



and 



both deduced from the 



Fig. 6. — Diagram Showing Method 
of Determining Tangent Oifsets 
for Arcs of over 90°. 



formula, y = r ± \lr^ — d?. 

Assuming the value of d = AG^ 
to locate the point H^ GrH = y^ = r 
— 'slr^ — (i^ and to locate the point 
iai=y^=r+ \l7^ - d\ 
By giving to d the values between and AF^ the various 
values for the tangent offsets are obtained. 

In staking out the center line of a curvilinear tunnel the 
greatest accuracy is required, since a very small error will throw 
the work out and cause 
serious trouble. At the 
beginning the excava- 
tion is conducted as 
closely as may be to the 
line of the curve, and 
as soon as it has pro- 
gressed far enough the 
tangent AT, Fig. 7, is 
ranged out. At ^ a 
point is located over 
wliich to set the instru- 
ment, and the distance 
AB is measured for the 
purpose of finding the ordinate of the right angle triangle GAB. 
Now OA = r, AB = d, and <^ = angle AB 0. Then : Tang. 
r 

* = 5- 




Frcr. 7. 



Method of Laying Out the Center Line of 
Curvilinear Tunnels. 



DETERMINING THE CENTER LINE 15 

Doubling the value of ^ and making the angle ABC = 2cf>, 
the line BC will be fixed and the point C located by taking 
AB = BC. On 56^ the ordinates are laid off to locate the curve. 
Prolong CB so that CD = CB. Then the portion of the curve 
CE is symmetrical with CE^ and the ordinates used to locate 
EC may be employed to locate (7F, by laying them off in the 
reverse order. 

FORM AND DIMENSIONS OF CROSS-SECTION. 

In deciding upon the sectional profile of a tunnel two factors 
have to be taken into consideration: (1) The form of section 
best suited to the conditions, and (2) the interior dimensions of 
this section. 

Form of Section. — The form of the sectional profile of a tun- 
nel should be such that the lining is of the best form to resist 
the pressures exerted by the unsupported walls of the tunnel 
excavation, and these vary with the character of the material 
penetrated. These pressures are both vertical and lateral in 
direction ; the roof, deprived of support by the excavation, tends 
to fall, and the opposite sides for the same reason tend to slide 
inward along a plane more or less inclined, depending upon the 
friction and cohesion of the material. In some rocks the co- 
hesion is so great that they will stand vertically, while it may 
be very small in hwse earth which slides along a plane whose 
inclination is diiectly proportional to the cohesion. 

From the theory of resistance of profiles we know that the 
resistance of a line to exterior normal forces is directly propor- 
tional to its degree of curvature, and consequently inversely 
proportional to the radius of the curve. Hence the sectional 
profile of a tunnel excavated through hard rock, where there 
are no lateral pressures owing to the great cohesion of the ma- 
terial, and having to resist only the vertical pressure, should 
])e designed to offer thfe greatest resistance at its highest point, 
and the curve must, therefore, be sharper there, and may de- 
crease toward the ])ase. In (piicksand, mud, or other material 



16 



TUNNELING 



practically without cohesion, the pressures will all be normal 
to the line of the profile, and a circular section is the one best 
suited to resist them. These theoretical considerations have 
been proved correct by actual experience, and they may be 
employed to determine in a general way the form of section to 
be adopted. Applying them to very hard rock, they give us 
a section with an arched roof and vertical side walls. In softer 
materials they give us an elliptical section with its major axis 
vertical, and in very soft quicksands and mud they give us the 
circular section. These three forms of cross-section and their 
modifications are the ones commonly employed for tunnels. 
An important exception to this general practice, however, is 
met with in some of the underground city rapid-transit rail- 
ways built of late years, where a rectangular or box section is 
employed. These tunnels are usually of small depth, so that 
the vertical pressures are comparatively light, and the bending 
strains, which they exert upon the flat roof, are provided for by 
employing steel girders to form the roof lining. 

From what has been said it will be seen that it is impossible 
to establish a standard sectional profile to suit all conditions. 
The best one for the majority of conditions, and the one most 
commonly employed, is a polycentric figure in which the num- 
ber of centers and the 
length of the radii are 
fixed by the engineer to 
meet the particular con- 
ditions which exist. In 
a general way this form 
of center may be con- 
sidered as composed of 
two parts symmetrical 
in respect to the vertical 
axis. Fig. 8 shows such 
a profile, in which DH is the vertical axis. The section is 
unsymmetrical in respect to the horizontal axis GrE, The 




;:;c:r=4.---x:;' 




Fio. 8. — Diagram of Polycentric Sectional Profile. 



DETERMINING THE CENTER LINE 17 

upper part forming the roof arch is usually a semi-circle or 
semi-oval, while the lower part, comprising the side walls 
and invert or floor, varies greatly in outline. Sometimes the 
side walls are vertical and the invert is omitted, as shown by 
Fig. 9 ; and sometimes the side walls are inclined, with their 
bottoms braced apart by the invert, as shown by Fig. 10. In 
more treacherous soils the side walls are curved, and are con- 
nected by small curved sections to the invert, as shown by Fig. 



'''3'^ Fig. 10. Fig. 11. 

Figs. 9 to 11. — Typical Sectional Profiles for Tunnel. 

11. In the last example the side walls are commonly called 
skewbacks, and the lower part of the section is a polycentric 
figure like the upper part, but dissimilar in form. 

In a tunnel section whose profile is composed entirely of 
arcs the following conditions are essential : The centers of the 
springer arcs G-a and Ea\ Fig. 8, must be located on the line 
GE', the center of the roof arc hDh' must be located on the 
axis HD ; the total number of centers must be an odd number ; 
the radii of the succeeding arcs from G- toward D and E toward 
D must decrease in length, and finally the sum of the angles 
subtended by the several arcs must equal 180°. 

Dimensions of Section. — The dimensions to be given to the 
cross-section of a tunnel depend upon the purpose for which it 
is to be used. Whatever the purpose of the tunnel, the follow- 
ing three points have to be considered in determining the size 
of its cross-section: (1) The size of clear opening required; (2) 
the thickness of lining masonry necessary ; and (3) the decrease 
in the clear opening from the deformation of the lining. 

Railway tunnels may be built either to accommodate one or 



18 



TUNNELING 



two tracks. In single-track tunnels a clear space of at least 2^ 
ft. on each side should be allowed for between the tunnel wall 
and the side of the largest standard locomotive or car, and a 
clear space of at least 3 ft. should be allowed for between the 
roof and the top of the same locomotive or car. Since the roof 
of the tunnel is arch-shaped, to secure a clearance of 3 ft. at 
every point will necessitate making the clearance at the center 
greater than this amount. In double-track tunnels the same 
amounts of side and roof clearances have to be provided for, 
and, in addition, there has to be a clearance of at least 2 ft. 
between trains passing on the two tracks. Referring to Fig. 8, 
and assuming the line AB to represent the level of the tracks, 
then the ordinary dimensions in feet required for both single- 
and double-track tunnels are as follows : — 





Height, D. F. 
Feet. 


Width, G. E. 
Feet. 


Height, C.F. 
Feet. 


Height, C.H. 
Feet. 


Single track . . . 
Double track . . . 


17.6 to 18 
26.6 to 28 


16.5 to 18 

26.6 to 28 


6 to 7.4 
6.3 to 6.9 


i to i AB 
i to i AB 



The thickness of the masonry lining to be allowed for varies 
Avith the material penetrated, as will be explained in a succeed- 
ing chapter where the dimensions for various ordinary condi- 
tions are given in tabular form. The lining masonry is subject 
to deformation in three ways : by the sinking of the whole 
masonry structure, by the squeezing together of the side walls 
by the lateral pressures, and by the settling of the roof-arch. 
The whole masonry structure never sinks more than three or 
iour inches, and merits little attention. The movement of the 
iside walls towards each other, which may amount to three or 
four inches for each wall without endangering their stability, 
has, however, to be allowed for ; and similar allowance must be 
made for the settling of the roof-arch, which may amount to 
from nine inches to two feet. 



EXCAVATlNc; MACHINES AND KOCK DlllLLS 



19 



CHAPTER III. 

EXCAVATING MACHINES AND ROCK DRILLS 
EXPLOSIVES AND BLASTING. 



Earth-Excavating Machines. — Comparatively few of the labor- 
saving machines employed for breaking up and removing loose 
soil in ordinary surface excavation are used in tunnel excava- 
tion through the same material. Several forms of tunnel 
excavating machines have been tried at various times, but only 
a few of them have attained any measure of success, and these 
have seldom been employed in more than a single work. In 
the Central London underground railway work through clay a 
continuous bucket excavator (Fig. 12) was employed with 




Fig. 12.— Soft Ground Bucket Excavating Machine : Central l^ondon Underground Railway. 

considerable saving in time and labor over hand work, and in 
some recent tunnel work in America the contractors made 
quite successful use of a modified form of steam shovel. These 
are the most lecent attempts to use excavating machines in 
soft ground, and they, like all previous attempts, must be 
classed as experiments rather than as examples of common 
practice. The shovel, the spade, and the pick, wielded by 



20 ■ TUNNELING ' . 

hand, are the standard tools now, as in the past, for excavating 
soft-ground tunnels. 

Rock-Excavating^ Machines. — At one period during the work 
of constructing the Hoosac tunnel considerable attention was 
devoted to the development of a rock excavating, boring, or 
tunneling machine. This device was designed to cut a groove 
around the circumference of the tunnel thirteen inches wide 
and twenty-four feet in diameter by means of revolving cutters. 
It proved a failure, as did one of smaller size, eight feet diame- 
ter, tried subsequently. During and before the Hoosac tunnel 
work a number of boring-machines of similar character were 
experimented with at the Mont Cenis tunnel and elsewhere in 
Europe ; but, like the American devices, they were finally 
abandoned as impracticable. 

Hand Drills. — Briefly described, a drill is a bar of steel 
having a chisel-shaped end or cutting-edge. The simplest form 
of hand drill is worked by one man, who holds the drill in one 
hand, and drives it with a hammer wielded by his other hand. 
A more efficient method of hand-drill w^ork is, however, where 
one man holds the drill, and another swings the hammer or 
sledge. Another form of hand drill, called a churn drill, con- 
sists of a long, heavy bar of steel, which is alternately raised 
and dropped by the workman, thus cutting a hole by repeated 
impacts. 

In drilling by hand the workman holding the drill gives it a 
{>artial turn on its axis at every stroke in order to prevent 
wedging and to offer a fresh surface to the cutting-edge. For 
the same reason the chips and dust which accumulate in the 
drill-hole are frequently removed. The instruments used for 
this purpose are called scrapers or aippers, and are usually very 
simple in construction. A common form is a strong wire hav- 
ing its end bent at right angles, and flattened so as to make a 
sort of scoop by which the drillings may be scraped or hoisted 
out of the hole. It is generally advantageous to pour water 
into the drill-hole while drilling to keep the drill from heating. 



EXCAVATING :\2ACH1NES AND ROClv DRILLS 21 

Power Drills. — AVlien the conditions are such that use can 
be made of them, it is nearly always preferable to use power 
drills, on account of their greater speed of penetration and 
greater economy of work. Power drills are worked by direct 
steam pressure, or by compressed air generated by steam or 
wiiter power, and stored in receivers from which it is led to the 
drills through iron pipes. A great variety of forms of power 
drills are available for tunnel work in rock, but they can nearly 
all be grouped in one of two classes: (1) Percussion drills, and 
(2) Rotary drills. 

Percussion Drills. — The first American percussion drill 
was patented by ]\Ir. J. J. Couch of Philadelphia, Penn., in 
i\Iarch, 1849. In May of the same year, Mr. Joseph W. Fowle, 
Avho had assisted Mr. Couch in developing his drill, patented a 
percussion drill of his own invention. The Fowle drill was 
taken up and improved by INIr. Charles Burleigh, and was first 
used on the Hoosac tunnel. In Europe Mr. Cave patented 
a percussion drill in France in October, 1851. This invention 
was soon followed by several others ; but it was not until Som- 
meiller's drill, patented in 1857 and perfected in 1861, was used 
on the Mont Cenis tunnel, that the problem of the percussion 
drill was practically solved abroad. Since this time numer- 
ous percussion drill patents have been taken out in both 
America and Europe. 

A percussion drill consists of a cylinder, in which works a 
piston carrying a long piston rod, and which is supported in 
such a manner that the drill clamped to the end of the piston 
rod alternately strikes and is withdrawn from the rock as the 
piston reciprocates back and^ forth in the cylinder. Means are 
devised by wliich the pistf .. rod and drill turn slightly on their 
axis after each stroke, and also by which the drill is fed for- 
ward or advanced as the depth of the drill-hole increases. 
The drills of this type which are in most common use in 
America are the Inge rsoU-Serge ant and the Pand. There are 
various other makes in common use, however, which differ 



22 TUNNELING 

from the two named and from each other chiefly in the methods 
by which the valve is operated. All of these drills work either 
with direct steam pressure or with compressed air. Workable 
percussion drills operated by electricity are built, but so far 
they do not seem to have been able to compete commercially 
with the older forms. No attempt will be made here to make 
a selection between the various forms of percussion drills for 
tVmnel work, and for the differences in construction and the 
merits claimed for each the reader is referred to the makers of 
these machines. All of the leading makes will give efficient 
service. It goes almost without saying that a good percussion 
drill should operate with little waste of pressure, and should 
be composed of but few parts, which can be easily removed and 
changed. 

Brill Mountings. — For tunnel work the general European 
practice is to mount power drills upon a carriage moving on 
tracks in order that they may be easily withdrawn during 
the firing of blasts. Connection is made with the steam or 
compressed air pipes by means of flexible hose which can 
easily be attached or detached as the drill advances or when it 
is moved for repairs or during blasts. Two, four, and sometinies 
more drills are mounted and work simultaneously on a single 
carriage. In America it has been found that column mount- 
ings have been more successful for tunnel work than any other 
form. The column mounting made by the Ingersoll-Sergeant 
Drill Co. is shown by Fig. 13. In using this form of mounting 
no tracks or other special apparatus is required ; it is not 
necessary, as is the case with the carriage mounting, to remove 
the debris before resuming operations, but as soon as the blast- 
ing lias been finished and the smoke has sufficiently disap- 
peared the column can be set up and drilling resumed. 

Rotarjj Drills. — Rotary drilling machines, or more simply 
rotary drills, were first used in 1857 in the Mont Cenis tunnel. 
The advantages claimed for rotary drills in comparison with 
percussion drills are : (1) That less power is required to drive 



EXCAVATING MACHINES AND KOCK DRILLS 



2^^ 



the drill, and the power is better utilized ; (2) once the ma- 
chines work easily they do not require continual repairs, and 
(3) in driving holes of large size the interior nucleus is taken 




Fig. 13. — Culuinn Mountin;,' for Percussion Drill : Ingersoll-Sergeant Drill Co. 



away intact, thus reducing work and increasing the speed of 
drilling. Rotary drills are extensively used for geological, 
mining, well-driving, and prospecting purposes; ])nt they are 
very seldom eiii})lny(*(l in tmnicls in America, although success 



.24 TUNNELING 

fully used for this purpose in Europe. The reason they have 
not gained more favor among American tunnel builders is due to 
some extent perhaps to prejudice, but chiefly to the great cost 
of the machine as compared with percussion drills, and to the 
expense of diamonds for repairs. Those who advocate these 
machines for tunnel work point out, however, that under ordi- 
nary usage the diamonds have a very long life, — borings of 
700 lin. ft. being recorded without repairs to the diamonds. 

The form of rotary drill used chiefly for prospecting pur- 
poses is the diamond drill. This machine consists of a hollow 
cylindrical bit having a cutting-edge of diamonds, which is 
revolved at the rate of from two hundred to four hundred 
revolutions per minute by suitable machinery operated by steam 
or compressed air. The diamonds are set in the cutting-edge of 
the bit so as to project outward from its annular face and also 
shghtly inside and outside of its cylindrical sides (Fig. 14). 
When the drill rod with the bit at- 
tached is rotated and fed forward the 
bit cuts an annular hole into the rock ; 
the drillings being removed from the 
hole by a constant stream of water 
which is forced down through the hol- 

FlG. 14. — Sketch of Diamond i i -n i i • ji 

x^riii Bit. ^^^^ ^^^'^^^ ^od and emerges, carry uig the 

debris with it, up through the narrow 

space between the outside of the bit and the walls of the hole. 

There are various makes of diamond drills, but they all operate 

in essentially the same manner. 

The rotary drill principally employed in Europe in tunneling 
is the Brandt. Tlie cutting-edge of the Brandt drill consists of 
hardened steel teeth. The bit is pressed against the rock by 
hydraulic pressure, and usually makes from seven to eight revo- 
lutions per minute. Some of the water when freed goes 
through the hollow bit, keeping it cool, and cleaning the hole of 
debris. A water pressure of from 300 to 450 lbs. per square 
inch is required to operate these drills. Rotary rock-drills 




EXCAVATING MACHINES AND KOCK DRILLS 25 

may be mounted either on carriages or on columns for tunnel 
work. 

EXPLOSIVES AND BLASTING. 

When the holes are once drilled, either by hand or power 
drills, they are charged with explosives. The principal explo- 
sives employed in tunneling are gunpowder, nitroglycerine, and 
dynamite. 

Gunpowder Gunpowder is composed of charcoal, sulphur, 

and saltpeter in proportions varying according to the quality of 
the powder. For mining purposes the composition employed 
is 65 ^ saltpeter, 15 '/^ sulphur, and 20 yc charcoal. It is a black 
granulated powder having a specific gravity of 1.5 ; the black 
color is given by the charcoal ; and the grains have an angular 
form, and vary in size from J in. to | in. Good blasting 
powder should contain no fine grains, which may be detected 
by pouring some of the powder upon a sheet of white paper. 
The force developed by the explosion of gunpowder is not 
accurately known ; it depends upon the space in which it is 
confined. Diffei-ent authorities estimate the pressure at from 
15,000 lbs. per sq. in. in loose blasts to 200,000 lbs. per sq. in. 
in gunnery. Authorities also differ in opinion as to the 
character of the gases developed by the explosion of gun- 
powder, a matter of vital concern to the tunnel engineer, since 
they are likely to affect the health and comfort of his Avork- 
men. It may be assumed in a general way, however, that the 
oxygen of the saltpeter converts nearly all of the carbon of 
the charcoal into carbon dioxide, a portion of which combines 
with the potash of the saltpeter to form carbonate of potash, 
the remainder continuing in the form of gas. The sulphur is 
converted into sulphuric acid, nnd forms a sulphate of potash, 
which by reaction is decomposed into hyposulphite and sul- 
phide. The nitrogen of the salt])eter is almost entirely evolved 
in a free state ; and the carl)on not having l)een wholly l)urnt 
into carbonic acid, there is a [proportion of carbonic oxide. 



26 TUNNELING 

Nitroglycerine Nitroglycerine is one of the modern explo- 
sives used as a substitute for gunpowder. It is a fluid pro- 
duced by mixing glycerine with nitric and sulphuric acids ; it 
freezes at +41° F., and burns very quietly, developing carbonic 
acid, nitrogen, oxygen, and water. By percussion or by the 
explosion of some substances, such as capsules of gunpowder 
or fulminate of mercury, nitroglycerine produces a sudden 
explosion in which about 1,250 volumes of gases are pro- 
duced. The pressure of these gases has been calculated at 
26,000 atmospheres, or 324,000 lbs. per sq. in. Nitroglycerine 
explodes very easily by percussion in its normal state, but with 
great difficulty when frozen ; hence, in America, at the begin- 
ning of its use, it was transported only in a frozen state. When 
dirty, nitroglycerine undergoes a spontaneous decomposition 
accompanied by the development of gases and the evolution 
of heat, which, reaching 388° F., causes it to explode. Not- 
withstanding the enormous pressures which nitroglycerine de- 
velops, it is very seldom used in its liquid state, but is mixed 
with a granular absorbent earth composed of the shells of 
diatoms. The fluid undergoes no chemical change by being 
absorbed, and explodes, freezes, and burns under the same con- 
ditions as in the fluid state. 

Dynamite The credit of rendering nitroglycerine available 

for the purposes of the engineer by mixing it with a granular 
absorbent is due to Albert Nobel of Stockholm, Sweden, who 
named the new material dynamite. The nitroglycerine in 
dynamite loses very little of its original explosive power, but 
is very much less easily exploded by percussion, and can be 
employed in horizontal as well as vertical holes, which was, of 
course, not possible in its liquid state. Dynamite must contain 
'at least 50 fc of nitroglycerine. Some manufacturers, instead of 
diatomaceous earth, use other absorbents which develop gases 
upon explosion and increase the force of the explosion. These 
mixtures are classed under the general name of false dyna- 
mites. A great many varieties of dynamite are manufactured^ 



EXCAVATING MACHINES AND ROCK DRILLS 27 

and each manufacturer usually makes a number of grades to 
which he gives special names. Dynamite for railway work, 
tunneling, and mining contains about 50 "^ of nitroglycerine ; for 
quarrying about 35 ^/, and for blasting soft rocks about 30 f. 
It is sold in cylindrical cartridges covered with paper. 

Storage of Explosives In diiving tunnels through rock 

large quantities of explosives must be used, and it is necessary 
to have some safe place for storing them. In many States 
there are special laws governing the transportation and storage 
of explosives ; where there is no regulation by law the engineer 
should take suitable precautions of his own devising. It is 
best to build a special house or hut in one of the most con- 
cealed portions of the work and away from the tunnel, and 
protect it with a lightning-rod and from fire. Strict orders 
should be given to the watchman in charge not to allow persons 
inside with lamps or fire in any foini, and smoking should be 
prohibited. The use of hammers for opening the boxes 
should be prohibited ; and dynamite, gunpoAvder, and fulminate 
of mercur}^ should not be stored together in the same room. 
A quantity of dynamite for two or three days' consumption 
may be stored near the entrance of the tunnel in a locked box, 
the keys of which are kept by the foreman of the work. 
When dynamite has been frozen the engineer should provide 
some arrangement by which it may be heated to a temperature 
not exceeding 120° F., and absolutely forbid it being thawed 
out on a stove or by an open fii-e. 

Fuses When gun[)owder is used in tunneling it is ignited 

by the Blickford match. This match, or fuse as it is more 
commonly called, consists of a small rope of yarn or cotton 
having as a core a small continuous thread of fine gunpowder. 
To protect the outside of the fuse from moisture it is coated 
with tar or some other impervious substance. These fuses are 
so well made that they burn very uniformly at the rate of 
about 1 ft. in 20 seconds, hence the moment of explosion can 
be pretty accurately fixed beforehand. Blickford matches 



28 TUNNELING 

have the ohjection for tunnel work of burning with a bad odor, 
especially when they are coated with tar, and to remedy this 
many others have been invented. Those of Rzika and Franzl are 
the best known of these. The former has many advantages, but 
it burns too quickly, about 3 ft. per second, and is expensive ; 
tlie latter consists of a small hollow rope filled with dynamite. 

Blickford matches cannot be used to explode dynamite, the 
use of a cartridge being required. These cartridges are small 
copper cylinders containing fulminate of mercury. They may 
be attached to the end of the Blickford match, which being 
ignited the spark travels along its length until it reaches the 
copper cylinder, where it explodes the fulminate of mercury, 
which in turn explodes the dynamite. Blasts may also be fired 
by electricity, which, in fact, is the most common and the 
preferable method, because several blasts can be fired simulta- 
neously, and because the current is turned on at a great dis- 
tance, thus affording greater safety to the workmen. 

The method of electric firing generally employed in America 
is known as the connecting series method, and consists in firing 
several mines simultaneously. The ends of the wires are 
scraped bare, and the wire of the first hole of Ihe series is 
twisted together with the wire of the second hole, and so on ; 
finally the two odd wires of the first and last holes are connected 
to two wires of a single cable or to two separate cables extend- 
ing to some safe place to which the men can retreat. Here the 
two cable wires are connected by binding screws to the poles of 
a battery, or sometimes to a frictional electric machine. The cur- 
rent passes through the wires, making a spark at each break, and 
so fires the fulminate of mercury, which explodes the dynamite. 

Simultaneous firing by electricity by utilizing the united 
strength of the blasts at the same instant secures about 10 ^o 
greater efficiency from the explosives. Another advantage 
of electric firing is that in case of a missfire of any one of the 
holes there is slight possibility of explosion afterwards, and the 
place can be approached at once to discover the cause. 



EXCAVATING MACHINES AND KOCK DRILLS 29 

Tamping. — Tamping is the material placed in the hole above 
the explosive to prevent the gases of explosion from escaping 
into the air. Tamping generally consists of clay. AVlien gun- 
powder is used the clay must be well rammed with a wooden 
tool, and paper, cotton, or some other dry material must be 
phiced betw^een the moist clay and the powder. When dyna- 
mite is used it is not necessary to ram the tamping, since the 
suddenness of the explosion shatters the rock before the clay 
can be driven from the hole. 

A few experienced men should be appointed to fire the 
blasts. These men should give ample warning previous to the 
blast in order that all machinery and tools which might be 
injured by flying fragments may be removed out of danger, and 
so that the workmen may seek safety. When all is ready they 
should fire the blasts, keeping accurate count of the explosions 
to ensure that no holes have missed fire, and should call the 
workmen back when all danger is over. In case any hole has 
missed fire it should be marked by a red lamp or flag. 

Nature of Explosions. — Wlien the explosives are ignited a 
sudden development of gases results, producing a sudden and 
violent increase of pressure, usually accompanied by a loud 
report. The energy of the explosion is exerted in all directions 
in the form of a sphere having its center at the point of explo- 
sion, and the waves of energy lose their force as the distance 
from this central point increases. The energy of the explosion 
at any point in the sphere of energy is, therefore, inversely 
proportional to the distance of this poirft from the center of 
explosion. In the vicinity of the center of explosion the gases 
have sufficient power to destroy the force of cohesion and 
shatter the rock ; further on, as they lose strength, they only 
destroy the elasticity of the material and produce cracks ; and 
still further away they only produce a shock, and do not affect 
the material. Within the sphere of energy there are, therefore, 
three other concentric spheres: the first one being where 
cohesion is destroyed, the second where elasticity is overcome, 



30 TUKNELING 

and the third where the shock is transmitted by elasticity. 
When the latter sphere comes below the surface, the gases 
remain inside the rock ; but when the surface intersects either 
of the other two spheres, the gases blow up the rock, forming a 
cone or crater, whose apex is at the point of explosion, and 
which is called the blasting-cone. The larger the blasting-cone 
is, the greater is the amount of rock broken up ; and the object 
of the engineer should, therefore, always be so to legulate the 
depth of the hole and the quantity of explosive as to secure the 
largest possible blasting cone in each case. Experiments are 
required to determine the most efficient depth of hole, and 
quantity of explosive to be employed, since these differ in 
different kinds of rock, with the position of the rock strata, 
etc. ; but in ordinary practice, the depths of the holes are made 
from 1^ ft. to 2 ft. in the heading and upper portion of the 
tunnel, when drilled by hand; and from 3 ft. to 5 ft. when 
drilled by power drills. In the lower portion of the profile, the 
holes are made deeper, from 3 ft. to 4 ft. when drilled by 
hand, and exceeding 5 ft. when drilled by power. The dis- 
tance of the holes apart should be about equal to the diameter 
of the blasting-cone ; as a general rule it is assumed that the 
base of the blasting-cone has a diameter equal to twice the 
depth of the hole. The following table gives the average 
number of holes required in each part of the excavation for the 
St. Gothard tunnel : 

NO. OF PART* NAME OF PART NO. OF HOLES 

1. Heading 6 to 9 

2. Right wiug of heading . 3 to 5 

3. Left wing of heading 3 to 5 

4. Shallow trench with core 2 

5. Deepening of trench to floor 6 to 9 

6. Narrow mass of core to left 3 

7. Greater mass of core to left 6 to 9 

8. Culvert 1 

Total section 30 to 43 

* The location of the parts numbered is shown by Fig. 15, p. 32. 



EXCAVATING MACHINES ^VND KOCK DIULLS 31 

The quantity of explosives required for blasting depends 
upon the quality of the rock, since the force of the explosives 
must overcome the cohesion of the rock,>\vliich varies with its 
nature, and often differs greatly in rocks of the same kind and 
composition. The quantity of explosives required to secure 
the greatest efficiency in blasting any particular rock may be 
determined experimentally, but in practice it is usually deduced 
by the following rules: (1) The blasting force is directly pro- 
portional to the weight of the explosives used, and (2) the bulk 
of the blasted rock is proportional to the cube of the depth of 
the holes. It is usually assumed, also, that the explosive 
should fill at least one-fourth the depth of the hole. 



32 



TUNNELING 



CHAPTER IV. 

GENERAL METHODS OF EXCAVATION: SHAFTS 
CLASSIFICATION OF TUNNELS. 



A NUMBEE of different modes of procedure are followed in 
excavating tunnels, and each of the more important of these 
will be considered in a separate chapter. There are, however, 
certain characteristics conmion to all of these methods, and 
these will be noted briefly here. 

Division of Section. — It may be asserted at the outset that 
the whole area of the tunnel section is not ordinarily excavated 
at one time, but that it is removed in sections, and as each 

section is excavated it is thoroughly 
timbered or strutted. The order in 
which these different sections are 
excavated varies with the method of 
excavation, and it is clearly shown 
for each method in succeeding chap- 
ters. As a single example to illus- 
trate the proposition just made, the 
division of the section and the se- 
quence of excavation adopted at the 
St. Gothard tunnel is selected (Fig. 
15). The different parts of the 
section were excavated in the order numbered ; the names given 
to each part, and the number of holes employed in breaking it 
down, are given by the table on page 30. Whatever method is 
employed, the work always begins by driving a heading, which 
is the most difficult and expensive part of the excavation. All 
the other operations required in breaking down the remainder 




Fig. 15.— Diagram Showing Sequence 
of Excavation for St, Gothard 
Tunnel. 



GENERAL METHODS OF EXCAVATION 33 

of the tunnel section are usually designated by the general 
term of enlargement of the profile. The various operations of 
excavation may, therefore, be classified either as excavation 
of the heading or enlargement of the profile. 

Excavation of the Heading. — There is considerable confusion 
among the different authorities regarding the exact definition 
of the term "heading" as it is used in tunnel work. Some 
authorities call a small passage driven at the top of the profile 
a heading, and a similar passage driven at the bottom of the 
profile a drift ; others call any passage driven parallel to the 
tunnel axis, whether at the top or at the bottom of the profile, 
a drift; and still others give the name "heading" to all such 
passages. For the sake of distinctness of terminology it seems 
preferable to call the passage a heading when it is located at 
the top of the profile, and a drift when it is located near the 
bottom. 

Headings and drifts are driven in advance of the general 
excavation for the following purposes : (1) To fix correctly 
the axis of the tunnel; (2) to allow the work to go on at 
different points without the gangs of laborers interfering with 
each other; (3) to detect the nature of material to be dealt with 
and to be ready in any contingency to overcome any trouble 
caused by a change in the soil ; and (4) to collect tlie water. 
The dimensions of headings in actual practice vary according 
to the nature of the soil through which they are driven. As 
a general rule they should not be less than 7 ft. in height, so as 
to allow the men to work standing, and have room left for the 
roof strutting. The width should not be less than 6 ft., to 
allow two men to work at the front, and to give room for 
the material cars without interfering with the wall strntting. 
Usually headings are made 8 ft. wide. The length of headings 
in practice varies according to circumstances. In very long 
tunnels through hard rock the headings are sometimes ex- 
cavated from 1000 ft. to 2000 ft. in advance, in order that they 
may meet as soon as possible and the rnnging of the center line 



34 



TUNNELING 



be verified, and so that as great an area of rock as possible may 
be attacked at the same time in the work of enlarging the 
profile. In short tunnels, where the ranging of the center line 
is less liable to error, shorter headings are employed, and in soft 
soils they are made shorter and shorter as the cohesion of the soil 
decreases. When the material has too little cohesion to stand 
alone, the tops and sides of the heading require to be supported 
by strutting. To prevent caving at the front of the heading, 
the face of the excavation is made inclined, the inclination 
following as near as may be the natural slope of the material. 

Enlargement of the Profile. — Tl:e enlargement of the profile 
is accomplished by excavating in succession several small 

prisms parallel to the heading, and 
its full length, which are so located 
that as each one is taken out the 
cross-section of the original heading 
is enlarged. The number, location, 
and sequence of these prisms vary 
in different methods of excavation, 
and are explained in succeeding 
chapters where these methods are 
described. To direct the excava- 
tion so as to keep it always within 
the boundaries of the adopted pro- 
file, the engineer first marks the center line on the roof of the 
heading by wooden or metal pegs, or by some other suitable 
means by which a plumb line may be suspended. He next 
draws to a. large scale a profile of the proposed section; and 
beginning at the top of the vertical axis he draws horizontal 
lines at regular intervals, as shown by Fig. 16, until they inter- 
sect the boundary lines of the profile, and designates on each 
of these lines the distance between the vertical axis and the 
point where it intersects the profile. It is evident that if the 
foreman of excavation divides his plumb line in a manner corre- 
sponding to the engineer's drawing, and then measures horizon- 




Fig. 16. — Diagram ShoAving Manner 
of Determining Correspondence of 
Excavation to Sectional Profile. 



GENERAL METHODS ()F EXCAVATION 



35 



tally and at right angles to the vertical center plane of the 
tunnel the distance designated on the horizontal lines of the 
drawing, he will have located points on the profile of the sec- 
tion, or in other words have established the limits of excava- 
tion. 

In the excavation of the Croton Aqueduct for the water 
supply of New York city, an instrument called a polar pro- 
tractor was used for determining the location of the sectional 




Fig. 17. — Polar Protractor for Determining Profile of Excavated Cross-Section. 

profile. This instrument consists of a circular disk graduated 
to degrees, and mounted on a tripod in such a manner that 
it may be leveled up, and also have a vertical motion and a 
motion about the vertical axis. The construction is shown 
clearly l)y Fig. 17. In use the device is mounted with its 
center at the axis of the tunnel. A light wooden measuring- 
rod tapering to a point, shod with brass and graduated to feet 
and hundredths of a foot, lies upon the wooden arm or rest, 
which revolves upon the face of the disk, and slides out to 



36 TUNNELING 

a contact with the surface of the excavation at such points 
as are to be determined. If the only information desired is 
whether or not the excavation is sufficient or beyond the es- 
tablished lines, the rod is set to the proper radius, and if it 
swings clear the fact is determined. If a true copy of the 
actual cross-section is desired, the rod is brought into contact 
with the significant points in the cross-section, and the angles 
and distances are recorded. 

The general method of directing the excavation in enlarging 
the profile by referring all points of the profile to the vertical 
axis is the one usually employed in tunneling, and gives good 
results. It is considered better in actual practice to have the 
excavation exceed the profile somewhat than to have it fall 
short of it, since the voids can be more easily filled in with 
riprap than the encroaching rock can be excavated during the 
building of the masonry. In tunnels where strutting is neces- 
sary the excavation must be made enough larger than the 
finished section to provide the space for it. In soft-ground 
tunnels it is also usual to enlarge the excavation to allow for 
the probable slight sinking of the masonry. The proper allow- 
ance for strutting is usually left to the judgment of the fore- 
man of excavation, but the allowance for settlement must be 
fixed by the engineer. 

SHAFTS. 

Shafts are vertical walls or passages sunk along the line of 
the tunnel at one or more points between the entrances, to 
permit the tunnel excavation to be attacked at several different 
points at once, thus greatly reducing the time required for 
excavation. Shafts may be located directly over the center 
of the tunnel or to one side of it, and, while usually vertical, 
are sometimes inclined. During the construction of the tunnel 
the shafts serve the same purpose as the entrances ; hence they 
must afford a passageway for the excavated materials, which 



GENERAL METHODS OF EXCAVATION 37 

have to be hoisted out, and also for the construction tools and 
materials which have to be lowered down them. They must 
also afford a passageway for workmen, draft animals, and for 
pipes for ventilation, water, compressed air, etc. The character 
of this trathc indicates the dimensions required, but these de- 
pend also upon the method of hoisting employed. Thus, when 
a windlass or horse gin is used, and the materials are hoisted 
in buckets of small dimensions, the dimensions of the shaft may 
also be small; but when steam elevators are employed, and the 
material is carried on cars run on to the platform of the elevator, 
large dimensions must be given to the shaft. Generally the 
parts of the shaft used for different purposes are separated by 
partitions. The elevator for workmen and the various pipes 
are placed in one compartment, while the elevator for hoisting 
the excavated material and lowering construction material is 
placed in another. 

Shafts may be either temporary or permanent. They are 
temporary when they are filled in after the tunnel is completed, 
and permanent when they are left open to supply ventilation 
to the tunnel. Permanent shafts are usually made circular, and 
lined with brick, unless excavated in very hard and durable 
rock. When sunk for temporary use only, shafts are usually 
made rectangular with the greater dimension transverse to the 
tannel. They are strutted with timber. A pump is generally 
located at the bottom of the shaft to collect the water which 
seeps in from the sides of the shaft and from the tunnel 
excavation. The dimensions of this pump will of course vary 
with the amount of water encountered, as will also the capacity 
of the pump for forcing it up and out of the shaft, which has 
always to be kept dry. 

The majority of engineers prefer to sink shafts directly 
over the center line of the tunnel. Side shafts are employed 
chiefly by French engineers. The chief advantage of the 
former method is the great facility which it affords for hoisting 
out the materials, while in favor of the latter method is the 



38 TUNNELING 

non-interference of the shaft with the operations inside the 
tunnel. Were it not that the side shaft requires tiie intro- 
duction of a transverse gallery connecting it with the tunnel, 
it would be on the whole superior to the center shaft ; but the 
side gallery necessitates turning the cars at right angles, and 
consequently the use of a very sharp curve or a turntable to 
reach the shaft bottom, which is a disadvantage that may 
outweigh its advantages in some other respects. It is impos- 
sible to state absolutely which of these methods of locating 
shafts is the best ; both present advantages and disadvantages, 
and the use of one or the other is usually determined more by 
the local conditions than by any general superiority of either. 

When side shafts are employed they are sometimes made 
inclined instead of vertical. This form is used Avhen the depth 
of the shaft is small. By it the hauling is greatly simplified, 
since the cars loaded at the front with excavated material can 
be hauled directly out of the shaft and to the dumping-place, 
surmounting the inclined shaft by means of continuous cables. 
The short galleries connecting the side shafts with the tunnel 
proper usually have a smaller, section than the tunnel, but are 
excavated in exactly the same manner. Another form of side 
shaft sometimes used is one reaching to the surface when 
the tunnel runs close to the side of cliff, as is the case with 
some of the Alpine railway tunnels. 

CLASSIFICATION OF TUNNELS. 

Tunnels are classified in various ways, but the most logical 
method would appear to be a grouping according to the quality 
of the material through which they are driven ; and this method 
will be adopted here. By this method we have first the fol- 
lowing general classification : (1) Tunnels in hard rock ; (2) 
tunnels in ordinary loose soil; (3) tunnels in quicksand; 
(4) open-cut tunnels; and (5) submarine tunnels. It is hardly 
necessary to say that this classification, like all others, is simply 



GENERAL METHODS OF EXCAVATION 39 

an arbitrary arrangement adopted for the sake of order and 
convenience in treating the snbject. 

Tunnels in Hard Rock. — With the numerous hibor-saving 
methods and machines now available, hard rock is perhaps the 
safest and easiest of all materials through which to drive a 
tunnel. Tunnels through hard rock may be excavated, either 
by a drift or by a heading. The difference depends upon 
whetlier the advance gallery is located close to the floor or 
near the soffit of the section. 

Tunnels in Loose Soils. — In driving tunnels through loose 
soils many different methods have been devised, which may be 
grouped as follows: (1) Tunnels excavated at the soffit — 
Belgian method; (2) tunnels excavated along the perimeter 
— (jerman method; (3) tunnels excavated in the whole sec- 
tion — English and Austrian methods ; (4) tunnels excavated 
in two halves independent of each other — Italian method. 

(1) Excavating the tunnel by beginning at the soffit of 
the section, or by the Belgian method, is the method of tunnel- 
ing in loose soils most commonly employed in Europe at the 
present time. It consists in excavating the soffit of the 
section first ; then building the arch, which is supported upon 
the unexcavated ground ; and finally in excavating the lower 
portion of the section, and building the side walls and 
invert. 

(2j In excavating tunnels along the perimeter an annular 
excavation is made, following closely the outUne of the sec- 
tional profile in which the lining masonry is built, after which 
the center core is excavated. In the German method two 
drifts are opened at each side of the tunnel near the bottom. 
Other drifts are excavated, one above the other, on each side 
to extend or heighten the first two until all the perimeter is 
open except across the bottom. The masonr}- lining is then 
l)uilt from the bottom upwards on each side to the crown of 
the arch, and then the center core is removed and the invert 
is built. 



40 TUNNELING 

(3) This method, as its name implies, consists in taking 
out short lengths of the whole sectional profile before begin- 
ning the building of the masonry^ In the English method 
the lengths of section excavated vary from 10 ft. to 25 ft. 
The masonry invert is built first, then the side walls, and 
finally the arch. Tiie excavators and the masons work alter- 
nately, the excavation being stopped while the masonry is 
being built, and vice versa. The Austrian method differs in 
two particulars from the English : the length of section opened 
is made great enough to allow the excavators to continue work 
ahead of the masons, and the side walls and roof are built 
before the invert. 

(4) Tlie Italian method is very seldom employed on account 
of its expensiveness, but it can often be used where the other 
methods fail. It consists in excavating the lower half of the 
section, and building the invert and side walls, and then filling 
the space between the walls in again except for a narrow 
passageway for the cars ; next the upper part of the section is 
excavated, as in the Belgian method, and the arch is built ; and 
finally the soil in the lower part is permanently removed. 

Tunnels in Quicksand. — Tunnels through quicksand are 
driven by one of the ordinary soft-ground methods after drain- 
ing away the water, or else as submarine tunnels. 

Open-Cut Tunnels. — Open-cut tunnels are those driven at 
such a small depth under the surface that it is more convenient 
to excavate an open cut, build tlje tunnel masonry inside it, 
and then refill the open spaces, than it is to carry on the work 
entirely underground. In firm soils the usual mode of opera- 
tion is to excavate first two parallel trenches for the side walls, 
then remove the core, and build the arch and the invert. In 
unstable soils, since the invert must be built first, it is usual 
to open up a single wide trench. In infrequent cases where 
a tunnel is desired in a place which is to be filled in, the 
masonry is built as a surface structure, which in due time is 
covered. 



GENERAL METHODS OF EXCAVATION 



41 



Submarine Tunnels. — The mode of procedure followed in 
excavating submarine tunnels depends upon whether the mate- 
rial penetrated is pervious or impervious to water. In imper- 
vious material any of the ordinary methods of tunneling found 
suitable may be employed. In pervious material the excava- 
tion may be accomplished either by means of compressed air 
to keep the water out of the excavation, or, by means of a 
shield closing the front of the excavation, or by a combination 
of these two methods. Tunnels on the river bed are built by 
means of coffer dams which inclose alternate portions of the 
work, or by sinking -a continuous series of pneumatic caissons 
and opening communication between them. 



Methods of 

Excavating 

Tlxnels. 



In hard rock. 



In loose soil. 



By drifts. 

By a heading. 

Bi/ upper half: 

the arch is built be- 
fore the side walls. 

By the perimeter: 
excavated and lined 
before the central 
nucleus is battered 
down. 

By whole section : 
the lining begins after 
the whole section is 
excavated. 

By halves : 

the lower half is ex- 
cavated, lined, and 
filled in again, fol- 
lowed by the work of 
the upper half. 



Belgian method. 

German method. 

I English method. 
Austrian method. 

> Italian method. 



In quicksand. 



Open-cut 
tunnels. 



Submarine 
tunnels. 



In resistant soils. 

In loose soils. 
Built up. 

At great depths under 
the river bed. 

At small depths 
under the river bed. 



On the river bed. 



By two lateral nar- 
row trenches. 

By one very large 
trench. 

By slices. 

By any method. 

r By shield. 
J By compressed air. 

I By shield and com- 

[ pressed air. 

r By coffer dams. 
^ By pneumatic cais- 

[ sons. 



42 TUNNELING 

The above diagram gives in compact form the classifica- 
tion of tunnels according to materials penetrated and methods 
of excavation adopted, which have been described more fully 
in the preceding paragraphs. It may be noted here again that 
this is a purely arbitrary classification, and serves mostly as a 
convenience in discussing the different classes of tunnels with- 
out confusion. 



TIMBERING OR STRUTTING TUNNELS 43 



CHAPTER V. 

METHODS OF TIMBERING OR STRUTTING 
TUNNELS. 



The purpose of timbering or strutting in tunnel work is to 
prevent the caving-in of the roof and side walls of the exca- 
vation previous to the construction of the lining. As the 
strutting has to resist all the pressures developed in the roof 
and side walls, which may be exceedingly troublesome and 
of great intensity in loose soils, its design and erection call 
for particular care. The method of strutting adopted depends 
upon the method of excavation employed ; but in ever}^ case 
the problem is not only to build it strong enough to withstand 
the pressures developed, but to do this as economically as 
possible, and with as little hindrance as may be to the work 
which is going on simultaneously and which will come later. 
Only the latter general problems of strutting peculiar to all 
methods of tunnel work will be considered here. For this 
consideration strutting may be classified according to the 
material of which it is built, under the heads of timber struc- 
tures and iron structures. 

TIMBER STRUTTING. 

Timber is nearly always employed for strutting in tunnel 
work. So long as it has the requisite strength, any kind of 
timber is suitable for strutting, since, it being only temporarily 
employed, its durability is a matter of slight importance. 
Timber with good elastic properties, like pine or spruce, is 
preferably chosen, since it yields gradually under stress, thus 



44 TUNNELING 

warning the engineer of the approach of danger ; while oak and 
other strong timbers resist until the last moment, and then 
yield suddenly under the breaking load. Soft woods, moreover, 
are usually lighter in weight than hard woods, which is a con- 
siderable advantage where so much handling is required in 
a restricted space. Round timbers are generally employed, 
since they are less expensive, and quite as satisfactory in other 
respects as sawed timbers. In the English and Austrian 
methods of strutting, which are described further on, a few 
of the principal struts are of sawed timbers. 

The various timbers of the strutting are seldom 

attached by framed joints, but wedges are used 

to give them the necessary 

^^^^^ bearing against each other. 



Where framed joints are em- 
ployed they are made of the 
simplest form usually by 



FIG. is.-joining Tunnel struts ployed thcv are made of the 

by Halving. i- ./ d 



11. , . . . . T , n -r^. -, r^ FiG.19.— Round 

halving the joining timbers, as shown by Iig. 18. Timber Post 
Fig. 19 shows a form of joint used where round f^^J^^ Cap Bear- 
posts carry beams of similar shape. The reason why 
it is possible to do away with jointed connections to such a 
great extent, is that the strains which the timbers have to 
resist are either compressive or bending strains, and because 
the timbers are so short that they do not require to be spliced. 
Strutting of Headings. — The method of strutting the head- 
ing that is employed depends upon the material through which 
the heading is driven. In solid rock strutting may not be 
required at all, or only for the purpose of preventing the 
fall of loose blocks from the roof, then vertical props are 
erected where required, or horizontal beams are inserted into 
the side walls, as shown by Fig. 20. These horizontal beams 
may be used singly at dangerous places, or they may be placed 
from 2 ft. to 3 ft. apart all along the heading. In the latter 
case they usually carry a lagging of planks, which may be 
placed at intervals or close together, and filled above with 



TEVIBERING OH STHUTTING TUNNELS 



45 



stone in case the roof of the excavation is very unstable. 
Phanks used in this manner are usually called poling-boards. 
Where the side walls as well as the roof require support, 





Fig. 20. — Ceiling Strutting for 
Tunnel Koofs. 



Fig. 21. — Ceiling Strutting with Sid( 
Post Supports. 



vertical side posts are employed to carry the roof beams, as 
shoAvn by Fig. 21 ; and, when necessary, poling-boards are 
inserted between these posts and the walls of the excavation. 

Frame Strutting. — In very loose soils not only the roof and 
side Avails, but also the floor of the heading require strutting. 





^Miik 


Miili/^li///iMii. 


^ 








1 








1 








i 






Fig. 22. — Sill, Side Post and Cap 
Cross FrMine Strutting. 




Fig. 23. — Keinforoed Cross Frame 
Strutting for Treaeherous Materials. 



In these cases frame strutting is employed, as shown ])y Fig. 
22. It consists simply of a rectangular frame ; at the top 
there is a crown l)ar supported })y two vertical side posts 



46 



TUNNELING 



setting on a sill laid across the bottom of the heading. These 
frames are spaced at close intervals, and carry longitudinal 
planks or poling-boards. The sill of the frame is sometimes 
omitted when the soil is stable enough to permit it, and in its 
place wooden footing blocks are substituted to carry the side 
posts. In soils where the pressures are great enough to bend 
the crown bar, a secondary frame is employed, as shown by 
Fig. 23, the two inclined roof members, or rafters, of which 
support the crown bar at the center. 

It is the more common practice in driving headings through 
soft soils to use inclined poling-boards to support the roof. 




Fig. 24. — Longitudinal Poling-Board Sys- 
tem of Roof Strutting. 



FiQ. 25. — Transverse Poling-Board System 
of Root Strutting. 



Fig. 24 shows one method of doing this. The method of 
operation is as follows : Assuming the poling-boards a and b 
to be in place, and supported by the frames A, B C^ as shown, 
the first step in continuation of the work is to insert the 
poling-board c over the crown bar of frame (7, and under the 
block m. Excavation is then begun at the top, and as fast as 
the soil is removed ahead of it the poling-board c is driven 
ahead until its rear end only slightly overhangs the crown bar 
of frame C, The remainder of the face of the heading is then 
excavated nearly to the front end of the poling-board c, and 
another frame is set up. By a succession of these operations 



TIMBERING OK STRUTTING TUNNELS 47 

the heading is advanced. The poling-boards at the sides of 
the heading are placed in a similar manner to the roof poling- 
boards. A second method of using inclined poling-boards is 
shown by Fig. 25. Here the poling-boards run transversely, 
and are supported by the arrangement of timbering shown. 
The chief advantage of using these inclined poling-boards, 
particularly in the manner shown by Fig. 24, is that the 
excavators work under cover at all times, and are thus safe 
from falling fragments or sudden cavings. 

Box Strutting. — In very treacherous soils, such as quick- 
sand, peat, and laminated clay, box strutting is commonly em- 
ployed. The method of building this strutting is to set up at 
the face of the work a rectangular frame, and use it as a guide 
in driving a lagging or boxing of horizontal planks into the 
soft soil ahead. These planks have sharp edges, and are driven 
to a distance of 2 ft. or 3 ft. into the face of the heading, so as 
to inclose a rectangular body of earth. This earth is excavated 
nearly to the ends of the planks, and then another frame is 
inserted ch)se up against the new face of the excavation, which 
supports the planks so that the remainder of the earth included 
by them may be removed. These two frames, with their plank 
lagging, constitute a " box ; " and a series of these boxes, one 
succeeding another, form the strutting of the heading. 

Strutting the Face. — In some cases it is found necessary 
to strut the face of the heading in order to prevent it from 
caving in. Tliis is generally done by setting plank vertically, 
and bracing them up by means of inclined props whose feet 
abut against the sill of the nearest cross frame. This strutting 
is erected while the workmen are placing the side and roof 
strutting, and is removed to pei'mit excavation. 

Full Section Timber Strutting. — For strutting the full section 
two forms of timbering are employed, known as the polygonal 
system and the longitudinal system. 

Longitudinal strutting consists of a timber structure so 
arranged as to have all the principal meml)ers supporting the 



48 



TUNNELING 



poling-boards parallel to the axis of the tunnel. This system 
of strutting is peculiar to the English method of tunnehng. 
The longitudinal timbers rest on this finished masonry at one 
end, and are carried on a cross frame or by props at the other 
end. At intermediate points the longitudinals are braced 
apart by struts in planes transverse to the tunnel axis. This 
construction makes a very strong strutting framework, since 
the transverse struts act as arch ribs to stiffen the longitu- 
dinals ; but the use of transverse poling-boards requires the 
excavation of a larger cross-section than is necessary when longi- 
tudinal poling-boards are employed, and this increases the 
cost both for the amount of earth excavated and the greater 
quantity of filling required. 

In polygonal strutting the main members are in a plane 
normal to the axis of the tunnel. They form a polygon whose 
sides follow closely the sectional profile of the excavation. 
These polygonal frames are placed at more or less short inter- 
vals apart, and are braced together by short longitudinal struts 
lying close to the sides of the excavation, and running from 
one frame to the next, and also by longer longitudinal members 
which extend over several frames. The polygonal system of 
strutting is peculiar to the Austrian method of tunneling, and 
is fully described in a succeeding chapter. One of its distinc- 
tive characteristics is that 
the poling-boards are in- 
serted parallel to the tunnel 
axis. Polygonal strutting 
is generally held to be 
stronger than longitudinal 
strutting under uniform 
loads, but it is more liable 
to distortion when the 
loads are unsym metrical. 
Strutting of Shafts. — Tunnel shafts are strutted both to 
prevent the caving-in of the sides and to divide them into 




IMG. 26. — Shaft with Single Transverse 
strutting. 



TIMBERING OR STRUTTING TUNNELS 



49 




Fig. 27. 



Rectangular Frame Strutting for Shafts. 



compartments. When the material penetrated is very compact, 
and caving is not likely, a single series of transverse struts, one 
above the otlier, running from the top to the bottom of the 
shaft, as shown by Fig. 26, is used to divide it into two com- 
partments. In softer material, where the sides of the shaft 
require support, Fig. 27 
shows a form of strutting 
commonly employed. It 
consists of vertical corner 
posts braced apart at inter- 
vals by f(mr horizontal struts 
placed close to the walls of 
the shaft. The longer side 
struts are also braced apart 
at the center by a middle strut which divides the shaft into 
two compartments. A lagging of vertical plank is placed 
between the walls of the shaft and the horizontal side struts. 
In very loose soils the form of strutting shown by Fig. 28 is 
employed. This is practically the same construction as is 
shown by Fig. 27, with the addition of an interior polygonal 

horizontal bracing in each 



half of the shaft. Referring 
to P^ig. 28, the timbers a, a, 
etc., are vertical and con- 
tinuous from the top to the 
bottom of the shaft; and 
the horizontal timbers, 5, J, 
etc., are spaced at more or 
less close intervals verti- 
cally. The lagging plank 
may be laid with spaces between them, or close together, or, 
in case of very loose material, with their edges overlapping. 
The manner of constructing the strutting is also governed by 
the stability of the soil. In firm soils it is possible to sink the 
shaft quite a depth without timbering, and the timbering can 




Fig. 28. — Reinforced Rectangular Frame Strut- 
ting for Sliafts in Treacherous Materials. 



50 TUNNELING ^ 

be erected in sections of considerable length, which is always 
An advantage, but in loose soils the timbering has to follow 
closely the excavation. 

The solid wall shaft struttings which have been described 
-are discontinued at the point where the shaft intersects the 
tunnel excavation ; and from this point to the floor of the 
tunnel an open timbering is employed, whose only duty is to 
support the weight of the solid strutting above. This timber- 
ing is made in various forms, but the most common is a timber 
truss or arch construction which spans the tunnel section. 

Quantity of Timber. — The quantity of timber employed in 
strutting a tunnel varies with the character of the material 
through which the tunnel is excavated: it is small for solid- 
Tock tunnels, and large for soft-ground tunnels. In the Bel- 
gian method of excavation a smaller quantity of timber is 
used than in any of the other ordinary methods. For single- 
track tunnels excavated by this method there will be needed 
on an average about 3 to 3 J cu. yds. of timber per lineal foot 
of tunnel. Practical experience shows that about four-fifths of 
the timber once used can be employed for the second time. 
In any of the methods in which the whole tunnel section is 
-excavated at once, the average amount of timber required per 
lineal foot is about 8.7 cu. yds. Of this amount about two- 
thirds can be used a second time. In the Italian method, in 
which the upper half and the lower half are excavated separately, 
-about 5 cu. yds. of timber are required per lineal foot of tunnel, 
about one-half of which can be employed a second time. For 
.quicksand tunnels the amount of timbering required per lineal 
ioot varies from 3 to 5 cubic yds. Shaft strutting requires 
from 1 to 1|- cu. yds. of timber per lineal foot. 

Dimensions of Timber. — The dimensions of the principal mem- 
bers composing the strutting of headings, full section, and 
ishafts, are given in Table I. The planks used for lagging 
or the poling-boards are usually from 4 ins. to 6 ins. wide, 
^th a length depending upon the method of strutting employed. 



TLMBERING OR STRUTTING TUNNELS 



51 



TABLE I. 

Showing Sizes of Various Timbers Used in Strutting Tunnels Driven 
Through Different Materials. 



Headings : 

Cap-pieces and vertical struts 

Sills 

Struts 

Distance apart of the frames in feet . . . 

Strutting of the tunnel, longitudinal strutting : 

Crown bars 

Props vertical or inclined supporting the crown 
bars 

Sills 

Cap-pieces or saddles 

Struts to stiffen the structure 

Distance apart of the frames (in feet) . . . 
Polygonal strutting : 

Cap-pieces and contour pieces 

Vertical struts on top 

Vertical struts below 

Intermediate sills 

Lower sills 

Raking props 

Distance apart of the frames (in feet) . . . 
Shafts : 

Horizontal beams forming the frame . . . . 

Transverse beams 

Vertical struts between the frames 

Struts to reenforce the frame . . .' . . . 

Distance apart of the strutting (in feet) . . . 



IlOClv. 


Soft Soils. 


ft 


i 


2 "S 


1 


S '8 


ins. 


ins. 


ins. 


ins. 


ins. 


G 


8 


10 


12 


14 






8 


10 


12 


5 


5 


G 


7 


8 


G 


4.5 


3 


2.6 


2.6 


12 


14 


14 






10 


12 


1^ 






8 


8 


10 






10 


12 


14 






6 


8 


10 






4.5 


4 


3 






8 


10 


12 


14 


16 


10 


12 


14 


16 


18 


12 


14 


16 


20 


24 


12 


14 


IG 


20 


24 






12 


16 


18 


10 


10 


10 


12 


12 


() 


4.5 


4 


3 


3 


8 


8 


10 


12 


14 


8 


8 


8 


10 


12 


8 


8 


10 


12 


12 




6 


8 


8 


8 


6 


4.5 


4 


3 


2.G 



IRON STRUTTING. 

In 1862 Mr. Rziha employed old iron railway rails for 
strutting the Naensen tunnel, and his example was successfully 
followed in several tunnels built later where timber was scarce 



62 



TUNNELING 





Pig. 29.— Strut- 
ting of Timber 
Posts and Rail- 
way Rail Caps. 



Fig. 30. — Strutting 
made entirely of 
Railway Rails. 



and expensive. The advantages which iron strutting is claimed 
to possess over the more common wooden structure are : its 
greater strength ; the smaller amount of space which it takes 
up ; and the fact that it does not wear out, and may, therefore, 
be used over and over again. 

Iron Strutting in Headings. — In strutting the headings the 
cross frames have a crown bar consisting of a section of old 
railway rail carried either by wood or iron side posts. When 
wooden side posts are used their 
upper ends have a dovetail mor- 
tise, and are bound with an iron 
band, as shown by Fig. 29. The 
base of the rail crown bar is set 
into the dovetail mortise and 
fastened by wedges. When iron 
side posts are employed they 
usually consist of sections of rail- 
way rails, and the crown bar is 
attached to them by fish-plate connections, as 
shown by Fig. 30. The iron cross frames are set up as the 
heading advances, and carry the plank lagging or poling-boards, 
exactly in the same manner as the timber cross frames previ- 
ously described. 

Full Section Iron Strutting. — The iron strutting devised by 
Mr. Rziha for full section work is shown by Fig. 31. Briefly 
described, it consists of voussoir-shaped cast-iron segments, 
which are built up in arch form. Fig. 32 shows the construc- 
tion of one of the segments, all of which are alike, with the 
exception of the crown segment, which has a mortise and 
tenon joint which is kept open by filling the mortise with sand. 
The segments are bolted together by means of suitable bolt- 
holes in the vertical flanges, and when fully connected form an 
arch rib of cast iron. This arch rib. A, Fig. 31, carries a series 
of angle or T-iron frames bent into approximately voussoir 
shape, as shown at B, Fig. 31. Above these frames are inserted 



TIMBERING OR STRUTTING TUNNELS 



53 




Fig. 31. 



• Rziha's Combined Strutting and Centering 
of Cast Iron. 



the poling-boards, running longitudinally, and spanning the 
distance between consecutive arch ribs. By removing the bent 
iron frames the cast-iron rib forms a center upon which to con- 
struct the masonry. Fi- 
nally, to remove the cast> 
iron rib itself, the sand 
is drawn out of the mor- 
tise and tenon joint in 
the crown segment, which 
allows the joint to close, 
and loosen the segments 
so that they are easily 
unbutted. 

The illustration, Fig. 
31, shows longitudinal 
poling-boards; more often 
longitudinal crown bars 

of railway rails span the space between connective arch ribs, 
and support transverse poling-boards. In building the masonry, 
work is begun at the bottom on each side, the bent iron frames 
being removed one after another to give room for the masonry. 

As each frame is removed, it is 
replaced with a sort of screw- 
jack to support the poling-boards 
until the masonry is sufficiently 
completed to allow their removal. 
The interior bracing of the arch 
rib shown at a a and h h consists 
of railway rails carried by brack- 
ets cast on to the segments. A 
similar bracing of rails connects the successive arch ribs. These 
lines of bracing serve to carry the scaffolding upon which the 
masons work in building the lining. 

Iron Shaft Strutting. — In soft-groimd shaft w^ork, the use of 
an iron struttiuc^, consistiiK^ of consecutive cast-iron rings, has 




Pig. .3?. —Cast-iron Segment of Rziha's 
Strutting and Centering. 



54 



TUNNELING 



sometimes been employed to advantage. Fig. 33 shows the 
construction of one of these rings, which, it will be seen, is com- 
posed of four segments connected to each other by means of 
bolted flanges. The holes shown in the circumferential web of 
the ring are to allow for the seepage from the earth side walls. 

The method of placing this 
cylindrical strutting is to 
start with a ring having a 
cutting-edge. By means 
of excavation inside the 
ring, and by ramming, 
the ring is sunk into the 
ground a distance equal to 
its height. Another ring 
is then fastened by special hooks on top of the first one, and 
the sinking continued until the second ring is down flush with 
the surface. A third ring is then added, and so on until the 
entire shaft is excavated and strutted. As in timber shaft 
strutting, the solid iron ring strutting is carried down only to 
the top of the tunnel section, and below this point there is an 
open timber or iron supporting framework. 




Fig. 33.— Cast-iron Segmental Strutting for 
Shafts. 



METHODS OF HAULING IN TUNNELS S5 



CHAPTER VI. 
METHODS OF HAULING IN TUNNELS. 



The transportation from one point to another within the 
tunnel and its shafts of any material, whether it is excavated 
spoil or construction material, is defined as hauling. In all 
engineering construction, the transportation of excavated 
materials, and materials for construction, constitutes a very- 
important part of the expense of the work; but hauling in 
tunnels where the room is very limited, and where work is 
constantly in progress over and at the sides of the track, is a 
particularly expensive process. Hauling in tunnels may be 
done either by way of the " entrances, or by way of the shafts, 
or by way of both the entrances and shafts. 

Hauling by Way of Entrances. — AVhen the hauling is done 
by the way of the entrances, the niaterials to be hauled are 
taken directly from the point 
of construction to the en- 
trances, or in the opposite di- 
rection, by means of special 
cars of different patterns. For 
general purposes, these differ- 
ent patterns of cars may be 
grouped into three classes, — 
platform-cars, dump-cars, and 
box-cars. Representative ex- 
amples of these several classes 

of cars are shown in Figs. 34 to 37 * inclusive, but it will be 
readily understood that there are many other forms. 

Briefly described, platform-cars (Fig. 34) consist of ii 

• Reproduced from catalogue of Arthur Koppel, New York, 






'f^.''"'" "^5 S"'^ "^''^' 



Fi(.. :j1. - ri;ah,.nn c'.ii iwi l-,;,,)i..l w« 



56 



TUNNELING 







Fig. 35. — Iron Dump-Car for 
Tunnel Work. 



wooden platform mounted on tracks, and they are usually em- 
ployed for the transportation of timber, ties, etc. Dump-cars 
are used in greater numbers in tunnel work than any other 
form. Fig. 35 shows a dump-car of metal construction, and 

Fig. 36 one constructed with a metal 
under-frame ^nd wooden box. These 
cars are made to run on narrow-gauge 
tracks, and usually have a capacity of 
about one to one and one-half cubic 
yards. Box-cars are more extensively 
employed in Europe for tunnel work 
than in America. Fig. 37 shows a 
typical European box-car for tunnel 
work. It is made either to run on narrow-gauge or standard- 
gauge tracks. 

It is usually desirable in tunnel work to employ cars of 
different forms for different parts of the work. In rock 
tunnels it is a common practice to use narrow-gauge cars of 
small size in the headings, and 
larger, broad-gauge cars for the 
enlargement of the profile. 
Where narrow-gauge cars are 
employed for all purposes, it will 
also be found more convenient 
to use platform-cars for handling 
the construction material, and 
dump-cars for removing the spoil. 
The extent to which it is desir- ^^^^.p^StP^'":^'^^ t^^ 

able to use cars of different forms fig- 36. -Wooden Dump-Car for Tunnel 

Work. 

will depend upon the character 

and conditions of the work, and particularly upon how far it is 

possible to install the permanent track. 

As a general rule, it is considered preferable to lay the 
permanent tracks at once, and do all the hauling upon them, 
so that as soon as the tunnel is completed, trains may pass 




METHODS OF HAULING IN TUNNELS 57 

through without delay. To what extent this may be done, or 
whether it can be done at all or not, depends upon the method 
of excavation and other local conditions. In soft>-ground 
tunnels excavated by the English or Austrian methods, 
it is quite possible to lay the permanent tracks at first, since 
the whole section is excavated at once, and the excavation is 
kept bat a little ahead of the completed tunnel. In rock 
tunnels, where the heading is driven far ahead of the com- 
pleted section, it is, of course, impossible to keep the perma- 
nent track close to the advance work, and narrow-gauge tracks 
must be laid in the heading. The same thing is true in soft^ 
ground tunnels driven by successive headings and drifts. In 
these cases, therefore, | 

where narrows-gauge i rffn" ,.>„ii,.i,., "^ffSr T 

tracks have to be used jscf-rf^-e^ ^^ ^ 

for some portions of 

the work anyway, the 

question comes up 

whether it is preferable 

to use temporary 

narrow-gauge tracks throughout, or to lay the permanent track 

as far ahead as possible, and then extend narrow-gauge tracks 

to the advance excavation. In the latter case it will, of course, 

be necessary to trans-ship each load from the narrow-gauge to 

the standard-gauge cars, or vice versa, which means extra cost 

and trouble. To avoid this, the method is sometimes adopted 

of laying a third rail between the standard-gauge rails, so that 

either standard- or narrow-gauge cars may be transported over 

the line. Whatever form the local conditions may require the 

system of construction tracks to assume, it may be set down as 

a general rule that the permanent tracks should be kept as far 

advanced as possil)le, and temporary tracks employed only 

where the permanent tracks are impracticable. 

The motive power employed for hauling in tunnels may be 

furnished by animals or by mechanical motors. Animal power 




Fig. 37 —Box-Car for Tunnel Work. 



58 TUNNELING 

is generally employed in short tunnels and in the advance 
headings and galleries. In long tunnels, or where the exca- 
vated material has to be transported some distance away from 
the tunnel, mechanical power is preferable, for obvious reasons. 
The motors most used are small steam locomotives, special 
compressed-air locomotives, and electric motors. Compressed 
air and electric locomotives are built in various forms, and are 
particularly well adapted for tunnel work because of their 
small dimensions, and freedom from smoke and heat. 

Hauling by Way of Shafts. — When the excavated material 
and materials of construction are handled through shafts, the 
operation of hauling may be divided into three processes : 
the transportation of the materials along the floor of the 
tunnel, the hoisting of them through the shaft, and the sur- 
face transportation from and to the mouth of the shaft. These 
three operations should be arranged to work in harmony with 
each other, so as to avoid waste of time and unnecessary han- 
dling of the materials. An endeavor should be made to avoid, 
if possible, breaking or trans-shipping the load from the time 
it starts at the heading until it is dumped at the spoil bank. 
This can be accomplished in two ways. One way is to hoist 
the boxes of the cars from their trucks at the bottom of the 
shaft, and place them on similar trucks running on the surface 
tracks. The other way is to run the loaded cars on to the ele- 
vator platform at the bottom, hoist them, and then run them 
on to the surface tracks. If the first method is employed, the 
car box is usually made of metal, and is provided at its top 
edges with hooks or ears to which to attach the hoisting cables. 
When the second method is used, the elevator platform has 
tracks laid on it which connect with the tracks on the tunnel 
floor, and also with those on the surface. 

Hoisting Machinery. — The machines most commonly em- 
ployed for hoisting purposes in tunnel shafts are steam hoisting 
engines, horse gins, and windlasses operated by hand. Wind- 
lasses and horse gins are rather crude machines for hoisting 



METHODS OF HAULING IN TUNNELS 59 

loads, and are used only in special circumstances, where the 
shaft is of small depth, when the amount of material to be 
hoisted is small, or where for any reason the use of hoisting 
engines is precluded. The steam hoisting engine is the stan- 
dard macliine for the rapid lifting of heavy vertical loads. 
Recently oil engines and electric hoists have also come to be 
used to some extent, and under certain conditions these ma- 
chines possess notable advantages. 

The construction of hand windlasses is familiar to every one. 
In tunnel work this device is located directly over the shaft, 
with its axis a little more than half a man's height, so that the 
crank handle does not rise above the shoulder line. To develop 
its greatest efficiency the hoisting rope is passed around the 
windlass drum so that the two ends hang down the shaft, and 
as one end descends the other ascends. A skip, or bucket, is 
attached to each of the rope ends ; and by loading the descend- 
ing skip with construction materials and the ascending skip 
with spoil, the two skip loads tend to balance each other, thus 
increasing the capacity of the windlass, and decreasing the 
manual labor required to operate it. Skips varying from 0.3 
cu. yd. to 0.5 cu. yd. are used. The horse gin consists of a 
vertical cylinder or drum provided with radial arms to which 
the horses are hitched, which revolve the cylinder by walking 
around it in a circle. The hoisting rope is rove around the 
drum so that the two ends extend down the shaft with skips 
attached, as described in speaking of the hand windlass. The 
power of the horse gin is, of course, much greater than that of a 
windlass operated by hand, skips of 1 cu. yd. capacity being 
commonly used. Horse gins are no longer economical hoisting 
machines, according to one prominent authority, when V 
(H-h20) > 5000, where V equals the volume of material to 
be hoisted, and H equals the height of the hoist, the weight of 
the excavated material being 2100 lbs. per cu. yd. As a gen- 
eral rule, however, it is assumed that it is not economical to 
employ horse gins with a depth of shaft exceeding 150 ft. 



60 TUNNELING 

As already stated, the most efficient and most commonly 
used device for hoisting at tunnel shafts is the steam hoisting 
engine. There are numerous builders of hoisting engines, each 
of which manufactures several patterns and sizes of engines. 
In each case, however, the apparatus consists of a boiler supply- 
ing steam to a horizontal engine which operates one or more 
rope drums. The engines are always reversible. They may 
be employed to hoist the skips directly, or to operate elevators 
upon which the skips or cars are loaded. In either case the 
hoisting ropes pass from the engine drum to and around ver- 
tical sheaves situated directly over the shaft so as to secure the 
necessary vertical travel of the ropes down the shaft. Where 
the shaft is divided into two compartments, each having an ele- 
vator or . hoist, double-drum engines are employed, one drum 
being used for the operations in one compartment, and the other 
for the operations in the other compartment. Where the work 
is to be of considerable duration, or when it is done in cold 
weather, more or less elaborate shelters or engine houses are 
built to cover and protect the machinery. 

Choice between the method of hoisting the skips directly, 
and the method of using elevators, depends upon the extent and 
character of the work. Where large quantities of material are 
to be hoisted rapidly, it is generally considered preferable to 
employ elevators instead of hoisting the skips directly. In 
direct hoisting at high speed, oscillations are likely to be pro- 
duced which may dash the skips against the sides of the shaft 
and cause accidents. The loads which can be carried in single 
skips are also smaller than those possible where elevators are 
used ; and this, combined with the slower hoisting speed required, 
reduces the capacity of this method, as compared with the use 
of elevators. Where elevators are employed, however, the plant 
required is much more extensive and costly ; it comprising not 
only the elevator cars with their safety devices, etc., but the 
construction of a guiding framework for these cars in the tun- 
nel shaft. For these various reasons the elevator becomes the 



METHODS OF HAULING IN TUNNELS 



61 



preferable hoisting device where the quantity of. material to be 
handled is large, where the shafts are deep, and where the work 
will extend over a long period of time ; but when the contrary 
conditions are the case, direct hoisting of the skips is generally 
the cheaper. The engineer has to integrate the various factors 
in each individual case, and 
determine which method will 
best fulfill his purpose, which 
is to handle the material at 
the least cost within the 
giv^en time and conditions. 
The construction of ele- 
vators for tunnel work is 
simple. The elevator car 
consists usually of an open 
framework box of timber and 
iron, having a plank floor on 
which car tracks are laid, 
and its roof arranged for 
connecting the hoisting cable 
(Fig. 38 *). Rigid construc- 
tion is necessary to resist the 
hoisting strains. The sides 
of the car are usually de- 
signed to slide against tim- 
ber guides on the shaft walls. 
Some form of safety device, 
of which there are several kinds, should be employed to pre- 
vent the fall of the elevator, in case the hoisting roj)e bi-eaks, 
or some mishap occurs to the hoisting machinery, which en- 
dangers the fall of the car. Speaking tubes and electric-bell 
signals should also be provided to secure communication be- 
tween the top and bottom of the shaft. 

* Reproduced from the catalogue of the Ledgerwood Manufacturing Company, New 
York. 




Fig. 38. — Elevator Car for Tunnel Shafts. 



62 TUNNELING 



CHAPTER VII. 

TYPES OF CENTERS AND MOLDS EMPLOYED 

IN CONSTRUCTING TUNNEL LININGS 

OF MASONRY. 



The masonry lining of a tunnel may be describea as con- 
sisting of two or more segments of circular arches combined 
so as to form a continuous solid ring of masonry. To direct 
the operations of the masons in constructing this masonry 
ring, templates or patterns are provided which show the exact 
dimensions and form of the sectional profile which it is de- 
sired to secure. These patterns or templates will vary in 
number and construction with the form of lining and the 
method of excavation adopted. Where the excavation is fully 
lined on all four sides, the masonry work is usually divided 
into three parts, — the invert or floor masonry, the side-wall 
masonry, and the roof-arch masonry. At least one separate 
pattern has to be employed in constructing each of these parts 
of the lining ; and they are known respectively as ground 
molds, leading frames, and arch centers, or simply centers. In 
the following paragraphs the form and construction usually 
employed for each of these three kinds of patterns is de- 
scribed. 

Ground Molds. — Ground molds are employed in building the 
tunnel invert. They are generally constructed of 3-inch plank 
cut exactly to the form and dimensions of the invert masonry 
as shown in Fig. 39. To permit of convenience of handling in 
a restricted space, they are generally made in two parts, which 
are joined at the middle by means of iron fish-plates and bolts. 
Either one or two ground molds may be employed. Where two 



TYPES OF CENTERS AND MOLDS 



63 




molds are used they are set up a short distance apart, and cords 
are stretched from one to the other parallel to the axis of the 
tunnel, by which the masons are guided in their work. Ex- 
treme care has to be taken in 
setting the molds to ensure that 
they are fixed at the proper 

grade, and are in a plane normal fig. 39. — Ground Moid for Constructing 
, . /• ,1 , 1 -fTTi Tunnel Invert Masonry. 

to the axis ot the tunnel. W here 

only one ground mold is employed, the finished masonry is 
depended upon to supply the place of the second mold, cords 
being stretched from it to the single mold placed the requisite 
distance ahead. The leveling and centering of the molds is ac- 
complished by means of transit and level. 

Two modifications of the form of ground mold shown by 
Fig. 39 are employed. The first modification is peculiar to 

the English method of 
excavation, and consists 
in combining the ground 
mold with the leading 
frame for the lower part 
of the side walls, as 
shown by Fig. 40. The 
second modification is 
employed where the two 
halves or sides of the 
invert are built separately, and it consists simply in using one- 
half of the mold shown by Fig. 39. When the last method of 
constructing the invert masonry is resorted to, extreme care has 
to be observed in setting the half-mold in order to avoid error. 

Leading Frames. — As already stated, leading frames are the 
patterns, or molds, used in building the side walls of the 1-ining. 
Like the ground mold they are usually built of plank ; one 
side being cut to the curve of the profile, and the other bei no- 
made parallel to the vertical axis of the tunnel section. The 
vertical side usually has some arrangement by which a plumb 




Fig. 40. 



-Combined Ground Mold and Leading Frame 
for Invert and Side Wall Masonry. 




64 TUNNELING 

bob can be attached, as shown by Fig. 41, to guide the work- 
men in erecting the frame. The combined leading frame and 
ground mold shown in Fig. 40 has already been described. 
The use of this frame is possible only w^here the 
masonry is begun at the invert and carried up on 
each side at the same time. This mode of con- 
struction is peculiar to the English method of 
tunneling ; in all other methods the form of sep- 
arate ground frame shown by Fig. 41 is employed. 
Fig. 41.— Lead- 'p]^g ground frames are lined in and leveled up by 

mg Frame for ^ i. J 

Constructing transit and level; and, as in setting the ground 

Side Wall Ma- « j_ i j. i x 

sonry. iramcs, care must be taken to secure accuracy m 

both direction and elevation. 

Arch Centers. — The template or form upon which the roof 
arch is built is called a center. Unlike the ground molds and 
leading frames, the arch centers have to support the weight of 
the masonry and the roof pressures during the construction of 
the lining, and they, therefore, require to be made strong. 
Owing to the fact that the pressures are indeterminate, it is 
impossible to design a rational center, and resort is had to those 
constructions which past experience has shown to work satis- 
factorily under similar conditions. In a general way it can 
always be assumed that the construction should be as simple 
as ]3ossible, that the center should be so designed that it can 
be set up and removed with the least possible labor, and that 
the different pieces of the framework and lagging should be as 
short as possible, for convenience in handling. 

Tunnel centers are usually composed of two parts, — a mold 
or curved surface upon which the masonry rests, and a frame- 
work which supports the mold. The curved surface or mold 
consists of a lagging of narrow boards running parallel to the 
tunnel axis, which rests upon the arched top members of tw^o 
or more consecutive supporting frames. The supporting frame 
is built in the form of a truss, and must be made strong enough 
to withstand the heavy superimposed loads, consisting of the 



TYPES OF CENTERS AND MOLDS 6o 

arch masonry during construction, and of the roof pressures 
which are transferred to the center when the strutting is 
removed to allow the masonry to be placed. The framework 
of the center is supported either by posts resting upon the floor 
of the excavation, or upon the invert masonry when this is 
built first, as in the English and Austrian methods, or it may 
be supported directly upon the ground where the arch masonry 
is built first, as in the Belgian method of tunneling. 

In describing the various methods of tunneling in succeed- 
ing chapters, the center construction and method of supporting 
the center peculiar to each will be fully explained, and only a 
few general remarks are necessary here. Centers may be classi- 
fied according to their construction and composition into plank 
centers, truss centers, and iron centers. 

One of the most common forms of plank centers is shown 
by Fig. 42. It consists of two 
ha If- polygons whose sides consist 
of 15 in. X 4 ft. planks having 
radial end-joints. These two half- 
polygons are laid one upon the 
other so that they break joints, as 
shown by the figure, and the ex- fig. 42.- piank center for constmct- 

i , ^ ing the Roof Arch. 

trados of the frame is cut to the 

true curve of the roof arch. The planks commonly used for 
making these centers are 4 ins. thick, making the total thick- 
ness of the center 8 ins. Plank centers of the construction 
described are suitable only for work where the pressures to be 
resisted are small, as in tunnels through a fairly firm rock, al- 
though there have been instances of their successful use in soft- 
ground tunnels. 

Where heavy loads have to be carried, trussed centers are 
generally employed, the trusses being composed of heavy square 
l)eams with scarfed and tenoned joints, reinforced by iron plates. 
Different forms of trusses are employed in each of the differ- 
ent methods of tunneling, and each of those is described in sue- 





66 TUNNELING 

ceeding chapters ; but they are generally either of the king-post 
^r queen-post type, or some modification of them. The king- 
post truss may be used alone, with 
or without the tie-beam, as shown 
by Fig. 43 ; but generally a queen - 
post truss is made to form the 
base of support for a smaller king 
post truss mounted on its top. 

Fig. 43.— Trussed Center for Constructing] L,, . 

the Roof Arch. This arrangement gives a trape- 

zoidal form to the center, which approaches closely to the arch 
profile. Owing to the character of the pressures transmitted to 
the center, the usual tension members can be made very light. 

The combined center and strutting system devised by Mr. 
Rziha has already been described in a previous chapter. In 
recent European tunnel work quite extensive use has also been 
made of iron centers consisting of several segments of curved 
I-beams, connected by fish-plate joints so as to form a semi- 
circular arch rib. The ends or feet of these I-beam ribs have 
bearing-plates or shoes made by riveting angles to the I-beams. 
Centers constructed in a similar manner, but made of sections 
of old railway rail, were used in carrying out the tunnel work 
on the Rhine River Railroad in Germany. The advantages 
olaimed for iron centers are that they take up less room, and 
that they can be used over and over again. 

Setting Up Centers. — According to the method of excava- 
tion followed in building the tunnel, the centers for building 
the roof arch may have to be supported by posts resting on the 
tunnel floor ; or where the arch is built first, as in the Belgian 
and Italian methods, they may be carried on blocking resting 
on the unexcavated earth below. Whichever method is em- 
ployed, an unyielding support is essential, and care must be 
taken that the centers are erected and maintained in a plane 
normal to the tunnel axis. To prevent deflection and twisting, 
the consecutive centers are usually braced together by longi- 
tudinal struts or by braces running to the adjacent strutting. 



TYPES OF CENTERS AND MOLDS 67 

Only skilled iind experienced workmen should be employed in 
erecting the centers ; and they should work under the immedi- 
ate direction of the engineer, who must establish the axis and 
level of each center by transit and level. 

Lagging. — By the lagging is meant the covering of narrow 
longitudinal boards resting upon the upper curved chords of the 
centers, and spanning the opening between consecutive centers. 
This lagging forms the curved surface or mold upon which the 
arch masonry is laid. When the roof arch is of ashlar masonry 
the strips of lagging are seldom placed neai'er together than 
the joints of the consecutive ring stones, but in brick arches 
they are laid close together. Besides the weight of the arch 
masonry, the lagging timbers support the short props which 
keep the poling-boards in place after the strutting is removed 
and until the arch masonry is completed. 

Striking the Centers. — The centers are usually brought to 
the proper elevation by means of wooden wedges inserted be- 
tween the sill of the center and its support, or between the 
bottom of the posts carrying the center and the tunnel floor. 
These wedges are usually made of hard wood, and are about 
6 ins. wide by -t ins. thick by 18 ins. long. To strike the center 
after the arch masonry is completed, these wedges are with- 
drawn, thus allowing tlie center to fall clear of the masonry. 
Usually the center is not removed immediately after striking, 
so that if the arch masonry fails the ruins will remain upon the 
center. The method of striking the iron center devised by Mr. 
Rziha has been described in the previous chapter on strutting. 



68 TUNNELING 



CHAPTER VIII. 
METHODS OF LINING TUNNELS. 



Tunnels in soft soils and in loose rock, and rock liable to 
disintegration, are always provided with a lining to hold the 
walls and roof in place. This lining may cover the entire 
sectional profile of the tunnel, or only a part of it, and it may 
be constructed of timber, iron, iron and masonry, or, more 
commonly, of masonry alone. 

Timber Lining. — Timber is seldom employed in lining 
tunnels except as a temporary expedient, and is replaced by 
masonry as soon as circumstances will permit. In the first 
construction of many American railways, the necessity for 
extreme economy in construction, and of getting the line open 
for traffic as soon as possible, caused the engineers to line 
many tunnels with timber, which was plentiful and cheap. 
Except for their small cost and the ease and rapidity with 
which they can be constructed, however, these timber linings 
possess few advantages. It is only the matter of a few years 
when the decay of the timber makes it necessary to rebuild 
them, and there is always the serious danger of fire. In 
several instances timber-lined tunnels in America have been 
burned out, causing serious delays in traffic, and necessitating 
complete reconstruction. Usually this reconstruction has con- 
sisted in substituting masonry in place of the original timber 
lining. In a succeeding chapter a description will be given of 
some of the methods employed in replacing timber tunnel 
linings with masonry. Various forms of timber lining are 
employed, of which Fig. 44 and the illustrations in the chapter 



METHODS OF LINING TUNNELS 



69 



discussing the metliods of relining timber-lined tunnels with 
masonry are typical examples. 

Iron Lining. — The use of iron lining for tunnels was intro- 
duced first on a large scale by i\lr. Peter William Barlow in 
1869, for the second tunnel under the River Thames at 
London, England, and it has greatly extended since that time. 
The lining of the second Thames tminel consisted of cylindrical 
cast-iron rings 8 ft. in diameter, the abutting edges of the 
successive rings being flanged and provided with holes for 
bolt fastenings. Each ring was made up of four segments, 




Cross Section. Longitudinal Section.. 

Fig. 44. — A Typical Form of Timber Lining for Tunnels. 

three of which were longer than quadrants, and one much 
smaller forming the "key-stone" or closing piece. These 
segments were connected to each other by flanges and bolts. 
To make the joints tight, strips of pine or cement and hemp 
yarn were inserted between the flanges. Since the construc- 
tion of the second Thames tunnel, iron lining has been em- 
ployed for a great many submarine tunneLs in England, 
Continental Europe, and America, some of them having a 
section over 28 ft. in diameter. Where circular iron linino- is 
employed, the bottom part of the section is leveled up witli 
concrete or brick masonry to carry the tracks, and the whole 



70 TUNNELING 

interior of the ring is covered with a cement plaster lining 
deep enough thoroughly to embed the interior joint flanges. 
In the succeeding chapter describing the methods of driving 
tunnels by shields several forms of iron tunnel lining are fully 
described. 

Iron and Masonry Lining. — During recent years a form of 
combined masonry and iron lining has been extensively em- 
ployed in constructing city underground railways in both 
Europe and America. Generally this form of lining is built 
with a rectangular section. Two types of construction are 
employed. In the first, masonry side walls carry a flat roof 
of girders and beams, which carry a trough flooring filled with 
concrete, or between which are sprung concrete or brick arches. 
Sometimes the roof framing consists of a series of parallel 
I-beams laid transversely across the tunnel, and in other cases 
transverse plate girders carry longitudinal I-beams. In the 
second type of construction the roof girders are supported by 
columns embedded in the side walls. Where the tunnel pro- 
vides for two or four tracks, intermediate column supports are 
in some cases introduced between the side columns. In this 
construction the roofing consists of concrete filled troughs or of 
concrete or brick arches, as in the construction first described. 
Examples of combined masonry and iron tunnel lining are 
illustrated in the succeeding chapter on tunneling under city 
streets. 

Masonry Lining. — The form of tunnel lining most commonly 
employed is brick or stone masonry. Concrete masonry lining 
has been employed in several tunnels built in recent years. The 
masonry lining may inclose the whole section or only a part of 
it. The floor or invert is the part most commonly omitted; 
but sometimes also the side walls and invert are both omitted, 
and the lining is confined simply to an arch supporting the 
roof. The roof arch, the side walls, and the invert compose 
the tunnel lining; and all three may consist of stone or brick 
alone, or stone side walls may be employed with brick invert 



METHODS OF LINING TUNNELS i 1 

and roof arch. Rubble-stone masonry is usually employed, 
except at the entrances, where the masonry is exposed to view. 
Here ashlar masonry is usually used. The stone selected for 
tunnel lining should be of a durable quality which weathers 
well. Where bricks are used they should be of good qual- 
ity. Owing to the comparative ease with which brick arches- 
can be built, they are generally used to form the roof arch, eveu 
where the side walls are of stone masonry. Masonry lining 
may be built in the form of a series of separate rings, or in the 
form of a continuous structure extending from one end of the 
tunnel to the other. The latter method of construction pro- 
duces a stronger structure ; but in case of failure by crush- 
ing, the damage done is likely to be more widespread than 
wliere separate rings are employed, one or two of which 
may fail without injury to the others adjacent to them. The 
CDiistruction is also somewhat simpler where separate rings are^ 
employed, since no provision has to be made for bonding the 
whole lining into a continuous structure. Where a series of 
separate rings is employed, the length of each ring runs fromi 
5 ft. up to 20 ft., it depending upon the character of the 
material penetrated, and the method of construction employed. 
F(^r the purpose of detailed discussion the construction of 
masonry lining may be divided into four parts, — the side-wall 
foundations, the side walls themselves, the roof arch, and the 
invert. 

Foundations. — In tunnels through rock of a hard and dur- 
able character the foundations for the side walls are usually 
laid directly on the rock. In loose rock, or rock hable to dis- 
integration, this method of construction is not generally a safe 
one, and the foundation excavation should be sunk to a depth 
at which the atmospheric influences cannot affect the founda- 
tion bed. In either case the foundation masonry is made 
thicker than that of the side walls proper, so as to distribute 
the pressure over a greater area, and to afford more room for 
adjusting the side-wall masonry to the proper profile. In 



72 



TUNNELING 



The form 




yielding soils a special foundation bed has to be prepared for 
the foundation masonry. In some instances it is found suffi- 
cient to lay a course of planks upon which the masonry is con- 
structed, but a more solid construction is usually preferred. 
This is obtained by placing a concrete footing from 1 ft. to 2 
ft. deep all along the bottom of the foundation trench, or in 
some cases by sinking wells at intervals along the trench and 
filling them with concrete, so as to form a series of supporting 
pillars. 

given to the foundation courses and lower 
portions of the side walls varies. Where 
a large bearing area is required, the back 
of the wall is carried up vertically as 
shown by the line AB, Fig. 45, otherwise 
the rear face of the wall follows the line of 
excavation A C. For similar reasons the front 
face of the wall may be made vertical, as at 
FG, or inclined, as at FR. The line FF 
indicates the shelf construction designed 
to sujDport the feet of the posts used to 
carry the arch centers during the construc- 
tion of the roof arch. 

Side Walls. — The construction of the side walls above the 
foundation courses is carried out as any similar piece of 
masonry elsewhere would be built. To direct the work and 
insure that the inner faces of the walls follow accurately the 
curve of the chosen profile, leading frames previously described 
are employed. 

Roof Arch. — For the construction of the roof arch, the 
centers previously described are employed. Beginning at the 
edges of the center on each side, the masonry is carried up a 
course at a time, care being taken to have it progress at the 
same rate on both sides, so that the load brought onto the 
centering is symmetrical. As soon as the centers are erected, 
the roof strutting is removed, and replaced by short props 



C H tj 

Fig . 45 . — Diagram 

Showing Forms 
Adopted for Side- 
Wall Foundations. 



METHODS OF LINING TUNNELS lb 

which rest on the higging of the centers and support the poling- 
boiirds. These props are removed in succession as the arcli 
masonry rises along the curve of the center, and the space 
between the top of the arch masonry and the ceiling of the 
excavation is lilled with small stones packed closely. The key- 
stone section of the arch is built last, by inserting the stones or 
bricks from the front edge of the arch ring, there being no 
room to set them in from the top, as is the practice in ordinary 
open-arch construction. The keying of the arch is an espe- 
cially difficult operation, and only experienced men skilled in the 
work should be employed to perform it. The task becomes 
one of unusual difficulty when it becomes necessary to join the 
arches coming from opposite directions. 

Invert — In all but one or two methods of tunneling, the 
invert is the last portion of the lining to be built. In the 
English method of tunneling, the invert is the first portion of 
the lining to be built, and the same practice is sometimes neces- 
sary in soft soils where there is danger of the bottoms of the 
side walls being squeezed together by the lateral pressures 
unless the invert masonry is in place to hold them apart. The 
ground molds previously described are employed to direct the 
construction of the invert masonry. 

General Observations. — In describing the construction of the 
roof arch, mention was made of the stone filling employed 
between the back of the masonry ring and the ceiling of the 
excavation. The spaces behind the side walls are filled in a 
similar manner. The object of this stone filling, whicli should 
be closely packed, is to distribute the vertical and lateral press- 
ures in tlie walls of the excavation uniformly over the lining 
masonry. As the masonry work progresses, the strutting 
employed previously to support the walls of the excavation has 
to be removed. This work requires care to prevent accident, 
and should be placed in charge of experienced mechanics wlio 
are familiar with its construction, and can remove it with the 
least damage to the timbers, so that they may be used again, 



74 TUNNELING 

and without endangering the fall of the roof or the caving of 
the sides by removing too great a portion of the timbers at one 
time. 

Thickness of Lining Masonry. — It is obvious, of course, that 
the masonry lining must be thick enough to support the press- 
ure of the earth which it sustains ; but, as it is impossible to 
estimate these pressures at all accurately, it is difficult to say 
definitely just what thickness is required in any individual case. 
Rankine gives the following formulas for determining the 
depths of keystone required in different soils : 
For firm soils, 



4' 



d = V/0.12-, 



and for soft soils, 



d 



\/0.48r 



where d = the depth of the crown in feet, r = the rise of the 
arch in feet, and s = the span of the arch in feet. Other 
writers, among them Professor Curioni, attempt to give rational 
methods for calculating the thickness of tunnel lining ; but they 
are all open to objection because of the amount of hypothesis 
required concerning pressures which are of necessity indetermi- 
nate. Therefore, to avoid tedious and uncertain calculations, 
the engineer adopts dimensions which experience has proven to 
be ample under similar conditions in the past. Thus we have 
all gradations in thickness, from hard-rock tunnels requiring 
no lining, and tunnels through rocks which simply require a 
thin shell to protect them from the atmosphere, to soft-ground 
tunnels where a masonry lining 3 ft. or more in thickness is 
employed. Table IT. shows the thickness of masonry lining 
used in tunnels through soft soils of various kinds. 

The thickness of the masonry lining is seldom uniform at 
all points, as is indicated by Table II. Figs. 46 and 47 show 
common methods of varying the thickness of lining at different 
points, and are self-explanatory. 



METHODS OF LINING TUNNELS 



75 



Side Tunnels. — When tunnels are excavated by shafts located 
at one side of the center line, short side tunnels or galleries are 
built to connect the bottoms of the shafts with the tunnel 
proper. These side tunnels are usually from 30. ft to 40 ft. 
long, and are generally made from 12 ft. to 1-4 ft. high, and 
about 10 ft. wide. The excavation, strutting, and lining of 
these side tunnels are carried on exactly as they are in the 
main tunnel, with such exceptions as these short lengths 
make possible. Table III. gives the thickness of lining used 
for side tunnels, the figures being taken from European 
practice. 




Figs. 46 and 47. 



Transverse Sections of Tunnels Showing Methods of Increasing the 
Thickness of the Lining at Different Points. 



Culverts. — The purpose of culverts in tunnels is to collect 
the water which seeps into the tunnel from the walls and shafts. 
The culvert is usually located along the center line of the 
tunnel at the bottom. In soft-ground tunnels it is built of 
masonry, and forms a part of the invert, but in rock tunnels it 
is the common practice to cut a channel in the rock floor of the 
excavation. Both box and arch sections are employed for 
culverts. The dimensions of the section vary, of course, with 
the amount of water which has to be carried away. The fol- 
lowing are the dimensions commonly employed : 



76 



tun:neling 



Kind of Culvert. 


Height in 
Feet. 


Width in 
Feet. 


Thicicness of 
Walls 
IN Feet. 


THICK>'ESS 
OF Gov EKING 

IN Feet. 


Box culvert .... 
Arch culvert .... 


1 to 1.5 
1 to 1.5 


Itol 5 
1 to 1.5 


0.8 to 1.2 
0.8 to 1.2 


0.3 
0.4 



It should be understood that the dimensions given in the 
table are those for ordinary conditions of leakage ; where larger 
quantities of water are met with, the size of the culverts has, 
of course, to be enlarged. To permit the water to enter the 
culvert, openings are provided at intervals along its side ; and 
these openings are usually provided with screens of loose stones 
which check the current, and cause the suspended material to 





Fig. 48. — Refuge Niche in St. Gothard Tunnel. 

be deposited before it enters the culvert. In cases where 
springs are encountered in excavating the tunnel, it is necessary 
to make special provisions for confining their outflow and con- 
ducting it to the culvert. In all cases the culverts should be 
provided with catch basins at intervals of from 150 ft. to 300 
ft., in which such suspended matter as enters the culverts is 
deposited, eaid removed through covered openings over each 
basin. At the ends of the tunnel the culvert is usually divided 
into two branches, one running to the drain on each side of the 
track. 

Niches. — In short tunnels niches are employed simply as 
places of refuge for trackmen and others during the passing of 
trains, and arc of small size. In long tunnels they ara made 



METHODS OF LINING TUNNELS 



77 



larger, and are also employed as places for storing small tools 
and supplies employed in the maintenance of the tunnel. 
Niches are simply arched recesses built into the sides of the 
tunnel, and lined with masonry ; Fig, 48 shows this construc- 
tion quite clearly. Small refuge niches are usually built from 
6 ft. to 9 ft. high, from 3 ft. to 6 ft. wide, and from 2 ft. 
to 3 ft. deep. Large niches designed for storing tools and 
supplies are made from 10 ft. to 12 ft. high, from 8 ft. to 10 ft. 
wide, and from 18 ft. to 24 ft. deep, and are provided with 



^ V/ ' 




■i::^7^ 


-^ ^ 






^i- 


1 ',:• 




■iii^.... 




•A'>v^ 


V 






■^ 


&. .. 






^fibVi 


fSfe^t: 






■'■"^-ri 


y;„ 3^^-i^^ 




\' 


V - 


^ 










gl^^ 


;psii^ 1 




R^^^^HM^H 








f 


^r,sa"^s2-^ 


V^^ _ ;: ;^ 






1 


^ i ^^.T.j - mum 






t 


;, 


SS^'^ '11^:2 


g'^S^j^'y^ 










^^^ 






'^^_ 


■ " J '"^i.-r4ik ^^v?** 


9^--;?:;*;^.;:.^ 












^;. 









Fig. 49. — East Portal of Hoosac Tunnel. 



doors. Refuge niches are usually spaced from 60 ft. to 100 ft. 
apart, while the larger storage niches may be located as far as 
3000 ft. apart. The niche construction shown by Fig. 47 is 
that employed on the St. Gothard tunnel. 

Entrances. — The entrances, or portals, of tunnels usually 
consist of more or less elaborate masonry structures, depending 
upon the nature of the material penetrated. In soft-ground 
tunnels extensive wing walls are often required to support the 
earth al)0ve and at the sides of the entrance : while in tunnels 



78 



TUNNELING 



through rock, only a masonry portal is required, tj give a finish 
to the work. Often the engineer indulges himself in an elabo- 
rate architectural design for the portal masonry. There is 
danger of carrying such designs too far for good taste unless 
cai;e;, is employed ; and on this matter the writer can do no better 
than to quote the remarks of the late Mr. Frederick W. Simms 
in his well-known " Practical Tunneling " : 

" The designs for such constructions should be massive to be suitable as 
approaches to works presenting the appearance of gloom, solidity, and strength. 
A light and highly decorated structure, however elegant and well adapted for 
other purposes, would be very unsuitable in such a situation ; it is plainness 
combined with boldness, and massiveness without heaviness, that in a tunnel 
entrance constitutes elegance, and, at the same time, is the most economical." 

Fig. 49 is an engraving from a photograph of the east portal 
of the Hoosac tunnel, which is an especially good design. 



TABLE II. 
Showing Thickness of Masonry Lining for Tunnels through Soft Ground. 



Character of Material. 


Keystone. 


Springers. 


Invert. 


Laminated clay, first variety . . 
Laminated clay, second variety . 
Laminated clay, third variety . . 
Quicksand 


Ft. 

2.15 to 3 
3 to 4.5 
4.5 to 6.5 
2 to 3.28 


Ft. 

2.75 to 3.5 
3.5 to 5.5 
5.5 to 8.1 
2 to 4.1 


Ft. 

1.6 to 2.5 
2.5 to 4 
4 to 4 5 
1.33 to 2.5 



TABLE III. 

Showing Thickness of Masonry Lining for Side Tunnels through 

Soft Ground. 



Character of Maticiual. 


Keystone. 


Springers. 


Invert. 


Laminated clay, first variety . . 
Laminated clay, second variety , 
Laminated clay, third variety 
(Quicksand 


Ft. 

1.6 to 2.3 
2.3 to 3 
3 to 4 
1 6 to 2.5 


Ft. 

1.8 to 3 
3 to 4.1 
4.1 to 5 
1.3 to 2 


Ft. 

1.5 to 2 
2 to 2 6 
2 6 to 3.21' 
1.3 to 2 



TUNNELS THKOUGH HARD llOCK 79 



CHAPTER IX. 

TUNNELS THROUGH HARD ROCK; GENERAL 
DISCUSSION; EXCAVATION BY DRIFTS. 
MONT CENIS TUNNEL. 



The present high development of hibor-saving machinery 
for excavating rock makes this material one of the safest and 
easiest to tunnel of any with which the engineer ordinarily has 
to deal. To operate this machineiy requii-es, however, the 
development of a large amount of power, its transmission to 
considerable distances, and, finally, its economical application 
to the excavating tools. The standard rock excavating ma- 
chine is the power drill, which requires either air or hydraulic 
pressure for its operation according to the special type em- 
ployed. Under present conditions, therefore, the engineer is 
limited either to air or water under compression for the trans- 
mission of his power. Steam-power may be employed directly 
to operate percussion rock drills ; but owing to the heat and 
humidity which it generates in the confined space where the 
drills work, and because of other reasons, it is seldom employed 
directly. Electric transmission, which offers so many advan- 
tages to the tunnel builder, in most respects is largely excluded 
from use by the failure which has so far followed all attempts 
to apply it to the operation of rock drills. As matters stand, 
therefore, the tunnel engineer is practically limited to steam 
and falling water for the generation of power, and to com- 
pressed air and hydraulic pressure for its transmission. 

Whether the engineer should adopt water-power or steam ^^ 
generate the power required tor his excavating machinery (h:^- 
pends upon their relative availahility, cost, and suital)ility to the 



80 TUNNELING 

conditions of work in each particular case. Where fuel is plen- 
tiful and cheap, and where water-power is not available at a 
comparatively reasonable cost, steam-power will nearly always 
prove the more economical ; where, however, the reverse con- 
ditions exist, which is usually the case in a mountainous 
country far from the coal regions, and inadequately supplied 
with transportation facilities, but rich in mountain torrents, 
water-power will generally be the more economical. In a suc- 
ceeding chapter the power generating and transmission plants 
for a number of rock tunnels are described, and here only a 
general consideration of the subject will be presented. 

Steam-Power Plant. — A steam-power plant for tunnel woik 
should be much the same as a similar plant elsewhere, except 
that in designing it the temporary character of its work must 
be taken into consideration. This circumstance of its tempo- 
rary employment prompts the omission of all construction 
except that necessary to the economical working of the plant 
during the period when its operation is required. The power- 
liouse, the foundations for the machinery, and the general con- 
struction and arrangement, should be the least expensive which 
will satisfy the requirements of economical and safe operation 
for the time required. It will often be found more economical 
as a whole to operate the machinery with some loss of economy 
during the short time that it is in use than to go to much 
greater expense to secure better economy from the machinery 
by design and construction, which will be of no further use 
after the tunnel is completed. The longer the plant is to be 
required, the nearer the construction may economically approach 
that of a permanent plant. As regards the machinery itself, 
whose further usefulness is not limited by the duration of any 
sins^le piece of work, true economy always dictates the purchase 
of the best quality. Speaking in a general way, a steam-power 
plant for tunnel work comprises a boiler plant, a plant of air 
compressors with their receivers, and an electric light dynamo. 
When hydraulic transmission of power is employed, the air 



TUNNELS THROUGH HARD liOCK 81 

coiupressors are replaced by high-pressure pumps ; and when 
electric hauling is employed, one or more dynamos may be re- 
quired to generate electricity for power purposes, as well as fur 
lio-htino-. In addition to the power generating machines proper, 
there must be the necessary piping and wiring for transmitting 
this power, and, of course, the equipment of drills and other 
machines for doing the actual excavating, hauling, etc. 

Reservoirs. — When water-power is employed, a reservoir 
has to be formed by damming some near-by mountain stream at 
a point as high as practicable above the tunnel. The provision 
of a reservoir, instead of drawing the water directly from the 
stream, serves two important purposes. It insures a continuous 
supply and constant head of water in case of drought, and also 
permits the water to deposit its sediment before it is delivered 
to the turbines. The construction of these reservoirs may be 
of a temporary character, or they may be made permanent 
structures, and utilized after construction is completed to sup- 
ply power for ventilation and other necessary purposes. In the 
first case they ai*e usually destroyed after construction is fin- 
ished. In either case, it is almost unnecessary to say, they 
should be built amply safe and strong according to good engi- 
neering practice in such works, for the duration of time which 
they are expected to exist. 

Canals and Pipe Lines. — For conveying the water from the 
reservoirs to the turbines, canals or pipe lines are employed. 
The latter form of conduit is generally preferable, it being 
both less expensive and more easily constructed than the 
former. It is advisable also to have duplicate lines of pipe to 
reduce the possibility of delay by accident or while necessary 
repairs are being made to one of the pipes. The pipe lines 
terminate in a penstock leading into the turbine chandjer, and 
provided with the necessary valves for controlling the admis- 
sion of water to the turbines. 

Turbines. — There are numerous forms of turbines on the 
market, l)Ut they may all be classed either as impulse turbines 



V 

82 tun:neling 

or as reaction turbines. Impulse turbines are those in which 
the whole available energy of the water is converted into 
kinetic energy before the water acts on the moving part of the 
turbine. Reaction turbines are those in which only a part of 
the available energy of the water is converted into kinetic 
•energy before the water acts on the moving vanes. Impulse 
turbines give efficient results with any head and quantity of 
water, but they give better results when the quantity of water 
varies and the head remains constant. Reaction turbines, on 
the contrary, give better results when the quantity of water 
remains constant and the head varies. These observations 
indicate in a general way the form of turbine which will best 
meet the particular conditions in each case. The number of 
turbines required, and their dimensions, will be determined in 
each case by the number of horse-power required and the 
quantity of water available. The power of the turbines is 
transmitted to the air compressors or pumps by shafting and 
gearing. 

Air Compressors. — An air compressor is a machine — usually 
driven by steam, although any other power may be used — by 
^vhich air is compressed into a receiver from which it ma}^ be 
piped for use. For a detailed description of the various forms 
of air compressors the reader should consult the catalogues of 
the several makers and the various text-books relating to air 
'Compression and compressed air. Air compressors, like other 
machines, suffer a loss of power by friction. The greatest loss 
of power, however, results from the heat of compression. 
When air is compressed, it is heated, and its relative volume 
is increased. Therefore, a cubic foot of hot air in the com- 
pressor cylinder, at say, 60 lbs. pressure, does not make a cubic 
foot of air at 60 lbs. pressure after cooling in the receiver. 
In other words, assuming pressure to be constant, a loss of 
volume results due to the extraction of the heat of compression 
;after the air leaves the compressor cylinder. To reduce the 
amount of this loss, air compressors are designed with means 



TUNNELS THROUGH HARD ROCK 83 

to extract the heat from the air before it leaves the com- 
pressor cylinder. Air compressors may first be divided into 
two ch\sses, according to the means employed for cooling the 
air, as follows: (1) Wet compressors, and (2) dry compress- 
ors. A wet compressor is one which introduces water directly 
into the cylinder during compression, and a dry compressor is 
one which admits no water to the air during compression. 
Wet compressors may be subdivided into two classes : (1) 
Those which inject water in the form of spray into the cylinder 
during compression, and (2) those which use a water piston 
for forcing the air into confinement. 

The following brief discussion of these various types of 
compressors is based on the concise practical discussion of 
Mr. W. L. Saunders, M. Am. Soc. C. E., in " Compressed Air 
Production." The highest isothermal results are obtained by 
the injection of water into the cylinders, since it is plain that 
the injection of cold water, in the shape of a finely divided 
spray, directly into the air during compression will lower the 
temperature to a greater degree than simply to surround the 
cylinder and parts by water jackets wliich is the means of cool- 
ing adopted with dry compressors. A serious obstacle to water 
injection, and that which condemns this type of compressor, is 
the influence of the injected water upon the air cylinder and 
parts. Even when pure water is used, the cylinders wear to 
such an extent as to produce leakage and to require reboring. 
The limitation to the speed of a compressor is also an important 
objection. The chief claim for the water piston compressor is 
that its piston is also its cooling device, and that the heat of 
compression is absorbed by tlie water. Water is so poor a 
conductor of lieat, liowever, that without the addition of sprays 
it is safe to say that this compressor has scarcely any cooling 
advantages at all so far as the cooling of the air during com- 
pression is concerned. The water piston compressor operates 
at slow Sliced and is expensive. Its only advantage is that it 
has no dead s})aces. In the dry compressor a sacrifice is made 



84 TUNNELING 

in the efficiency of the cooUng device to obtain low first cost, 
economy in space, light weight, higher speed, greater durability, 
and greater general availability. 

Air compressors are also distinguished as double acting and 
simple acting. They are simple acting when the cylinder is 
arranged to take in air at one stroke and force it out at the 
next, and they are double acting when they take in and force 
out air at each stroke. In form compressors may be simple or 
duplex. They are simple when they have but one cylinder, 
and duplex when they have two cylinders. A straight line or 
direct acting compressor is one in which the steam and air 
cylinders are set tandem. An indirect acting compressor is 
one in which the power is applied indirectly to the piston rod 
of the air cylinder through the medium of a crank. Mr. W. L. 
Saunders writes in regard to direct and indirect compression 
as follows : — 

" The experience of American manufacturers, which havS been more exten- 
sive than that of others, has proved the value of direct compression as distin- 
guished from indirect. By direct cpmpression is meant the application of 
power to resistance through a single straight rod. The steam and air cylinders 
are placed tandem. Such machines naturally show a low friction loss because 
of the direct application of power to resistance. This friction loss has been 
recorded as low as b%^ while the best practice is about 10% with the type which 
conveys the power through the angle of a crank shaft to a cylinder connected 
to the shaft through an additional rod." 

Receivers. — Compressed air is stored in receivers which are 
simply iron tanks capable of withstanding a high internal 
pressure. The purpose of these tanks is to provide a reservoir 
of compressed air, and also to allow the air to deposit its 
moisture. From the receivers the air is conveyed to the work- 
ings through iron pipes, which decrease gradually in diameter 
from the receivers to the front. 

Rock Drills. — The various forms of rock drills used in tun- 
neling have been described in Chapter III., and need not be 
considered in detail here except to say that American engi- 



TUNNELS THROUGH HARD ROCK 85 

ueers usually employ percussion drills, while European 
engineers also use rotary drills extensively. A comparison 
between these two types of drills was made in excavating the 
Aarlberg tunnel in Austria, where the Brandt hydraulic 
rotary drill was used at one end, and the Ferroux percussion 
drill was used at the other end. The rock was a mica-schist. 
The average monthly progress was 412 ft., with a maximum 
of 64:6 ft., with the rotary drills, and an average of 454 ft. with 
the percussion drill. 

Excavation. — Since considerable time is required to get the 
power plant established, the excavation of rock tunnels is often 
begun by hand, but hand work is usually continued for no 
longer a period than is necessary to get the power plant in 
operation. Generally speaking, the greatest difficulty is 
encountered in excavating the advanced drift or heading. 
Based on the mode of blasting employed, there are two methods 
of driving the advanced gallery, known as the circular cut 
and the center cut methods. In the first method a set of holes 
is first drilled near the center of the front in such a manner that 
they inclose a cone of rock ; the holes, starting at the perimeter 
of the base of the cone, converge toward a junction at its 
apex. Seldom more than four to six holes are comprised in 
this first set. Around these first holes are driven a ring 
of holes which inclose a cylinder of rock, and if necessary 
succeeding rings of holes are driven outside of the first ring. 
These holes are blasted in the order in which they are driven, 
the first set taking out a cone of rock, the second set enlarging 
this cone tD a cylinder, and the other sets enlarging this 
cylinder. These holes are seldom driven deeper than 4 or 5 
ft. In the center-cut method, which is the one commonly 
employed in America, the holes are arranged in vertical rows, 
and are driven from 15 ft. to 20 ft. deep. Figs. 50 to 53 
inclusive show the arrangement of the holes and the method 
OT blasting them. The two center rows of holes converge 
toward each other so as to take out a wedge of rock, l)ut the 



86 



TUNNELING 



others are bored " straight " or parallel with the vertical plane 
of the tunnel. 

The width of the advanced gallery or heading depends 
upon the quality of the rock. In hard rock American engi- 
neers give it tlie full width of the tunnel section; but this 
cannot be done in loose or fissured rock, which has to be sup- 
ported, the headings here being usually made about 8x5 ft. 
The wider heading is always preferable, where it is possible. 




Figs, 50 to 53. — Sketches Illustrating American Center-Cut Method of Blasting Tunnels. 

since more room is available for removing the rock, and deeper 
holes can be bored and blasted. 

With the preceding general discussion of tunneling through 
rock we may proceed to a detailed consideration of the con- 
struction of typical examples of rock tunnels. For this pur- 
pose the Mont Cenis and Simplon tunnels are selected as 
examples of rock tunnels driven by a drift, and the St. Goth- 
ard and Busk tunnels as examples of rock tunnels driven by 
headings. 



TUNNELS THROUGH HARD ROCK 



81 



EXCAVATION BY DRIFTS: MONT CENIS TUNNEL. 



General Description. — The method of tunneling throughr 
hard rock by drifts is preferred by European engineers. Botli 
the Mont Cenis tunnel built in 1857-70, and the great Siniplon 
tunnel now under construction, are examples of tunneling by 
drifts. In this method the sequence of excavation is shown 
diagram ma tically by Fig. 54. As soon as the top portion of 
the section has been opened, the roof arch is built with its feet 
resting on the tops of parts No. 4. These parts are removed 
by breaking down the outer portion 
between the sides of part No. 1 and 
the lines a b and a^ b^ first, and 
then by driving transverse cuts 
throuo'h to the sides of the section 
at intervals, and filling them with, 
the masonry of the side walls. 
These short sections or pillars of 
masonry serve to carry the arch 
while the rock between them is 
being excavated and the remainder 
of the side walls built. In hard 
rock the successive parts Nos. 1 to 
4 are driven several hundred feet in advance of each other. 

The drift is usually strutted by means of side posts carrying 
a cap-piece placed at intervals, and having a ceiling of longi- 
tudinal planks resting on the successive caps. In hard rock 
the roof of the section does not, as a rule, require regular 
strutting, occasional supports being placed at intervals to pre- 
vent the fall of isolated fragments. When the rock is disinte- 
grated or full of seams, a regular strutting may be necessary^ 
and this may be either longitudinal or polygonal in type. 
When longitudinal strutting is employed, a sill is laid across 
the roof of the diift, and upon this are set up two struts con- 




a q' 

Fig. 54, — Diagram Showing Se- 
quence of Excavations in Drift 
Method of Tunneling lioek. 



88 TUNNELING 

verging toward the top and supporting a cap-piece close to 
the roof. On this cap-piece are placed the first longitudinal 
crown bars carrying transverse poling-boards. Additional 
props standing on the sill and radiating outward are inserted 
as parts No. 3 are excavated. These- radial props carry 
longitudinal bars which in turn support transverse poling- 
boards. When polygonal strutting is used, it may have the 
construction described below as being employed in the Mont 
Cenis tunnel, or may take the form of three -or five segment 
arches of heavy timbers. 

The roof arch, usually of brick masonry, is built before the 
side w^Us, which are generally of rubble masonry, with its feet 
supported temporarily by the unexcavated rock below. Plank 
centers are usually employed, since the pressures they carry are 
usually limited to the weight of the masonry. The method 
of underpinning the roof arch with the side walls is that pecu- 
liar to the Belgian method of tunneling. The drain is usually 
constructed of brick masonry, and may be located at the center 
or at one side of the tunnel floor. 

Tunnels excavated by drifts enable simple means of hauling 
to be employed, and this is one of the reasons why the method 
finds so much favor with European engineers. The tracks 
are laid along the floor of the drift, and carry all the spoil 
from parts Nos. 2, 3, and 4, as well as from the front of the 
drift itself. As fast as the full section is completed, this single 
track in the drift is replaced by two tracks running close to 
the sides of the tunnel, or by a broad-gauge track with a third 
rail. 

Mont Cenis Tunnel — The Mont Cenis tunnel was the first 
of the great Alpine tunnels to be built. It is 7.9 miles long, 
and connects France and Italy by a double-track railway. Con- 
struction was begun in 1857, and the tunnel was opened for 
traffic in 1872. 

Material Penetrated. — The material penetrated by the ex- 
cavation consisted chiefly of limestone, calcareous schist, gneiss. 



TUNNELS THIIOUCni ilARI) IIDCK 



89 



and schistose sandstone. The stratification of the rock was 
nearly perpendiciUar to the axis of the tunnel, except at a few 
points where the strata intersected the axis at angles varying 
from 35° to 60°. 

Excavation. — The tunnel was driven exclusively from the 
ends by means of a drift. The diagram, Fig. ^b^ shows the 
order of the excavation, which began by the drift No. 1, whose 
dimensions were 9.5 x 8.5 ft., its roof reaching the line of the 
springers of the arch, and its floor being about 3 ft. higher than 
that of the tunnel. With the excavations of the part No. 2, 
the drift was widened on each side, 
except at the roof, and the floor of 
the tunnel was reached. Above the 
drift, the heading No. 3 was exca- 
vated, and when the parts No. 4 
were battered down the excavation 
of the upper portion of the tunnel 
section was completed, and the ma- 
sonry arch of the lining built. The 
parts No. 5 were afterward re- 
moved, and the side walls built up 
from foundations, and the arch un- 
derpinned. In the middle of the 
floor, the part No. 6 was excavated, and the culvert built. 

The excavation of the Mont Cenis tunnel was carried on 
by hand labor up to the year 1861, when the first drilling 
machine was employed. The drift, when excavated by hand 
labor, was blasted by means of many charges placed in holes no 
more than 1 J ft. deep, and very close together. When the per- 
forating machines were first used the same manner of boring 
numerous shaft-holes was followed, and the drift was excavated 
by a circular cut. Near the center 13 holes were driven, which 
formed the first round of blasting; close to sides 16 lioles were 
bored on each side for the second round ; 8 holes below and 13 
above the circular cut formed the tliird round; and close to the 



4-3 4 

5 I 5 

2. 2 

Z 

— r-6~i — 



Fig. 55. — Diagram Showing Se- 
quence of Excavation in Mont 
Cenis Tunnel. 



90 TUNNELING 

floor 5 more holes were bored for the fourth round, by which 
the floor of the drift was reached. The total number of the 
holes bored at the front of the drift varied from 70 to 80 ; their 
depth was 3^ ft. Three holes in the middle of those of the 
first round were made deeper so as to loosen the rock a little to 
facilitate the blasting of the succeeding rounds. Gunpowder 
was the only explosive used in the excavation of the Mont 
Cenis tunnel, both when the work was done by hand labor and 
by machines. 

The time required for boring the holes of the drift varied 
between 6 and 8 hours. From H to 2 hours were required for 
filling in the holes with explosives, and from 3 to 5 hours in 
removing the blasted rock, so that in 24 hours no more than 
two blasts were made at the front of the drift. The different 
excavations were made by various gangs following each other 
at an average distance of 900 ft. 

Power Plant. — The mechanical installation consisted of 
the Sommeilier air compressors built near the portals. The 
Sommeilier compressors, Mr. W. L. Saunders says, were oper- 
ated as a ram, utilizing a natural head of water to force air at 
80 lbs. pressure into a receiver. The column of water con- 
tained in the long pipe on the side of the hill was started and 
stopped automatically by valves controlled by engines. The 
weight and momentum of the water forced a volume of air with 
such a shock against the discharge valve that it was opened, 
and the air was discharged into the tank ; the valve was then 
closed, the water checked ; a portion of it was allowed to dis- 
charge, and the space was filled with air, which was in turn 
forced into the tank. Only 73 % of the power of the water was 
available, 27 % being lost by the friction of the water in the 
pij)es, valves, bends, etc. Of the 73 % of net work, 49.4 was 
consumed in the perforators, and 23.6 in a dummy engine 
for working the valves of the compressors and for special 
ventilation. 

The compressed air was conveyed from each end through a 



TUNNELS THROUGH HARD ROCK 91 

cast-iron pipe 7f in. in diameter, up to the front of the excava- 
tion. The joints of the pipes were made with turned faces, 
grooved to receive a ring of oakum which was tightly screwed 
and compressed into the joint. To ascertain the amount of 
leakage of the pipes, they and the tanks were filled with air 
compressed to 6 atmospheres, and the machines stopped ; after 
12 hours the pressure was reduced to 5.7 atmospheres, or to 
95 % of the original pressure. 

Sommeilier's percussion drilling machines were used in the 
excavation of this tunnel. They were provided with 8 or 10 
drills acting at the same time, and mounted on carriages running 
on tracks. These were withdrawn to a safe place during the 
blasting, and advanced again after the broken rock was removed 
from the front and the new tracks laid. 

JNIachine shops were built at both ends of the tunnel for 
building and repairing the drilling machines, bits, tools, etc. 
A gas factory was built at each end for lighting purpose. 

Strutting. — The roof of the drift was strutted by means of 
longitudinal planks supported by cap-pieces laid across the line 
of the tunnel and resting on vertical props close to the sides 
of the excavation. This strutting was necessitated in order to 
prevent the fall of the rock from the upper part of the section. 
For the upper portion of the profile no continuous strutting 
was required, but at places where the rock was fissured or 
disintegrated a polygonal strutting was employed. This 
consisted of a sill laid across the axis of the tunnel and just 
above the roof of the drift. On this sill two inclined props 
were placed supporting a cap-piece. Close to the feet of these 
two inclined props other props were inserted abutting against 
wooden blocks close to the faces of the excavation. These 
blocks were of trapezoidal shape, the smaller side being near 
the excavation, while the longer ones abutted against the 
props. Between two consecutive wooden blocks small l)eams 
were inserted as close as possible to the excavation, and in 
such a manner as to assume the form of a polygon. Planks 



92 TUNNELING 

were stretched longitudinally between the beams forming the 
polygons of the consecutive timber structures. 

Masonry. — After the upper portion of the tunnel section 
had been excavated, the arch was built with its feet resting 
upon heavy planks. For the construction of the arch light 
centers were used. The arch was made of brick, and rested on 
the unexcavated portions of the bench. When these were 
removed, pillars of rock from 6 to 8 ft. long were left at equal 
intervals between them. In the spaces left vacant, balks of 
timber were inserted in order to support the arch. In the 
space between the rock pillars the side walls were built up 
from foundation and the arch underpinned ; then the rock pillars 
were in their turn battered down, new timbers were inserted 
to support the arch, and the side walls were built and the arch 
underpinned. In this way the masonry of the lining was made 
continuous. At every 3,000 ft. large niches were built, while 
all along the line on both sides small sheltering niches were 
built 150 ft. apart. 

Hauling. — In the Mont Cenis tunnel all the hauling was 
done by horses. On the floor of the drift small tracks were 
placed, upon which ran the cars that removed the broken rock 
produced by blasting at the front. At the end of the drift the 
small cars dumped the rock into larger cars runnmg on the 
floor of the part No. 2 which was the tunnel floor. There a 
single track was laid, which was afterward switched into a 
double track where the full section of the tunnel was opened. 
The materials excavated from the upper portion of the profile, 
by means of openings left in the roof of the drift, were loaded 
directly on to the large cars running on the tunnel floor. 

Ventilation. — Ventilation was at first obtained by the air 
discharged from the drills, which exhausted from 250,000 to 
280,000 cu. ft. of fresh air every hour at the front. When this 
quantity was considered too small, a blower 2.5 ft. in diameter 
was employed. It was operated by a small compressed air 
motor, and the air was driven to the front through a 10 in. box 



TUNNELS THROUGH HARD ROCK 93 

conduit of square section. When the work was well advanced 
this apparatus was deemed to be insufficient, and the exhaust- 
ing bells described in the Chapter Ventilation were used, and 
operated by a powerful turbine, whose motive-power was a 
stream of 75 gallons of water per second with a head of 60 ft. 



94 TUNNELING 



CHAPTER X. 

TUNNELS THROUGH HARD ROCK (Continued). 
THE SIMPLON TUNNEL.* 



Before entering upon a description of the constructive 
details of this, the longest railway tunnel in the world, it may 
be well to give a general idea of the undertaking. Many 
schemes for the connection of Italy and Switzerland by a rail- 
way near the Simplon Road Pass have been devised, including 
one involving no great length of underground work, the line 
mounting by steep gradients and sharp curves. The present 
scheme, put forward in 1881 by the Jura-Simplon Ry. Co., con- 
sists broadly of piercing the Alps between Brigue, the present 
railway terminus in the Rhone Valley, and Iselle, in the 
gorge of the Diveria, on the Italian side, from which village 
the railway will descend to the existing southern terminus at 
Domo d'Ossola, a distance of about 11 miles. 

In conjunction with this scheme a second tunnel is pro- 
posed, to pierce the Bernese Alps under the Lotschen Pass 
from Mittholz to a point near Tnrtman in the Rhone Valley ; 
and thus, instead of the long detour by Lausanne and the Lake 
of Geneva, there will be an almost direct line from Berne to 
Milan via Thun, Brigue, and Domo d'Ossola. 

Starting from Brigue, the new line, running gently up 
the valley for IJ miles, will, on account of the proximity of 
the Rhone, which has already been slightly diverted, enter the 
tunnels on a curve to the right, of 1,050 ft. radius. At a 
distance of 153 yards from the entrance, the straight portion 

* Abstract from a paper read before the Institution of Civil Engineers by Charles B. 
Fox, Jan. 26, 1900. 



TUNNELS THROUGH HARD ROCK 95 

of the tunnel commences, and extends for 12 miles. The line 
then curves to the left with a radius of 1,311 ft. before emerging 
on the left bank of the Diveria. Commencing at the nortliern 
entrance, a gradient of 1 in 500 (the minimum for efficient 
drainage) rises for a length of 5h miles to a level length of 
550 yards in the center, and then a gradient of 1 in 143 de- 
scends to the Italian side. On the way to Domo d'Ossola one 
helical tunnel will be necessary, as has been carried out on the 
St. Gothard. There will be eventually two parallel tunnels, 
having their centers oQ ft. apart, each carrying one line of way; 
but at the present time only one heading, that known as No. 1, 
is being excavated to full size, No. 2 being left, masonry lined 
where necessary, for future developments. By means of cross 
headings every 220 yds. the problems of transport and ventila- 
tion are greatly facilitated, as will be seen later. As both 
entrances are on curves, a small " gallery of direction " is 
necessary, to allow corrections of alinement to be made direct 
from the two observatories on the axis of the tunnel. 

The outside installations are as nearly in duplicate as cir- 
cumstances will allow, and consist of the necessary offices, 
workshops, engine-sheds, power-houses, smithies, and tlie nu- 
merous buildings entailed by an important engineering scheme. 
Great care is taken that the miners and men working in the 
tunnel shall not suffer from the sudden change from the warm 
headings to the cold Alpine air outside ; and for this puipose 
a large building is in course of erection, where they will be 
able to take off their damp working clothes, have a hot and 
cold douche, put on a warm dry suit, and obtain refreshments 
at a moderate cost l)efore returning to their homes. Instead 
of each man having a locker in which to stow his clothes, a 
jjerfect forest of cords hangs down from the wooden ceiling, 
25 ft. above floor-level, each cord passing over its own pulleys 
and down the wall to a numbered belaying-pin. Each cord 
supports three hooks and a soap-dish, which, when loaded with 
their owner's property, are hauled up to the ceiling out of tlic 



96 TUNNELING 

Avay. There are 2,000 of these cords, spaced 1 ft. 6 ins. apart, 
one to each man. The engineers and foremen are more priv- 
ileged, being provided with dressing-rooms and baths, partitioned 
off from the two main halls. An extensive clothes washing 
and drying plant has been laid down, and also a large restau- 
rant and canteen. At Iselle, a magazine holding 2,200 lbs. of 
dynamite is surrounded and divided into two separate parts by 
earth-banks, 16 ft. high. The two wooden houses, in which 
the explosive is stored, are warmed by hot-water pipes to a 
temperature between 61° F. and 77° F., and are watched by 
a military patrol; but at Brigue a dynamite manufactory, 
started by an enterprising company at the time of the com- 
mencement of the works, supplies this commodity at frequent 
intervals, thereby avoiding the necessity of storing in such 
large quantities. This dynamite factory has been largely in- 
creased, and supplies dynamite to nearly all the mining and 
tunneling enterprises in Switzerland. 

Geological Conditions. — Before the Simplon tunnel was au- 
thorized, expert evidence was taken as to the feasibility of 
the project. The forecasts of the three engineers chosen, 
in reference to the rock to be encountered and its probable 
temperature, have, as far as the galleries have gone (an ag- 
gregate distance of nearly 2i miles), generally been found 
correct. At the north end, a dark argillaceous schist veined 
with quartz was met with, and from time to time beds of 
gypsum and dolomite have been traversed, the dip of the 
strata being on the whole favorable to progress, though timber- 
ing is resorted to at dangerous places. Water was plentiful 
at the commencement ; in fact, one inrush has not been stopped, 
and is still flowing down the heading. The total quantity of 
water flowing from the tunnel mouth is 16 gallons per second, 
of which 2 gallons per second are accounted for by the drilling 
machines. At Iselle, however, a very hard antigorio gneiss 
obtains, and is likely to extend for 4 miles. Very dry and 
very compact, it requires no timbering, and presents no great 



TUNNELS THIIOUCIH HARD ROCK 97 

difficulty to the powerful Brandt rock-drills, which work under 
a head of 3,:280 ft. of water. 

The tenipei-ature of the rock depends not only on the depth 
from the surface, but largely upon the general form of that sur- 
face combined with the conductivity of the rock. Taking 
these points into consideration with the experience gained from 
tlie construction of the St. Gothard tunnel, 95° F. was esti- 
mated as the probable maximum temperature, owing to the 
height of Monte Leone (11,660 ft.), which lies almost directly 
over the tunnel axis. 

Survey — After having determined upon the general position 
of the tunnels, taking into consideration the necessary gra- 
dients, the temperature of the rock, and a large bed of trouble- 
some gypsum on the north side, two fixed points on the 
proposed center line were taken, one at each entrance of tunnel 
No. 1, and the bearings of these two points, with reference to 
a triangulati on survey made in 1876, were calculated sufficiently 
accurately to determine, for the time being, the direction of 
the tunnel. In 1898, a new triangulation survey was made, 
taking in eleven summits, Monte Leone holding the central 
position. This survey was tied into that of the Wasenhorn 
and Faulhorn, made by the Swiss Government, and the accuracy 
was such that the probable error in the meeting of the two 
headings is only 6 cms. or 2^ ins. 

On the top of each summit is placed a signal, consisting of 
a small pillar of masonry founded on rock, and capped with a 
sharp pointed cone of zinc, 1 ft. 6 ins. high. An observatory 
was built at each end of the tunnel in such a position that three 
of the summits could be seen, a condition very difficult to fulfill 
on the south side owing to the depth of the gorge, the moun- 
tains on either side being over 7,000 ft. high. Having taken 
the angles to and from each visible signal, and therefrom having 
calculated the direction of the tunnel, it was necessary to fix, 
with extreme accuracy, sighting-points on the axis of the tunnel, 
in order to avoid sighting on to the surrounding peaks for each 



98 TUNNELING 

subsequent correction of the alinement of the galleries. To 
do this, a theodolite 24 ins. long and 2| ins. in diameter, 
with a magnifying power of 40 times, was set up in the observ- 
atory, and about 100 readings were taken of the angles between 
the surrounding signals and the required sighting-points. In 
this manner the error likely to occur was diminished to less 
than V. Thus at the north end two points were found about 
550 yds. before and behind the observatory, while on the south 
side, owing to the narrowness of the gorge, the points could 
only be placed at 82 yds. and 126 yds. in front. One of these 
sighting-points consists of a fine sciatch ruled on a piece of glass 
fixed in an iron frame, behind which is placed an acetylene 
lamp, — corrections of alinement are always done by night, — 
the whole being rigidly fixed into a niche cut in the rock and 
protected from climatic and other disturbing agencies by an 
iron plate. 

Method of Checkings Alinement. — The direction of heading 
No. 1 is checked by experts from the Government Survey De- 
partment at Lausanne about three times a year, and for this 
purpose a transit instrument is set up in the observatory. A 
number of three-legged iron tables are placed at intervals of 
1 mile or 2 miles along the axis of tunnel No. 1, and upon 
each of these is placed a horizontal plane, movable by means of 
an adjusting screw, in a direction at right angles to the axis, 
along a graduated scale. On this plane are small sockets, into 
which the legs of an acetylene lamp and screen, or of the 
transit instrument, can be quickly and accurately placed. The 
screen has a vertical slit, 3 ins. in height, and variable between 
^1 in. and ^V i^- i^ breadth, according to the state of the atmos- 
phere, and at a distance shows a fine thread of light. The 
instrument, having first been sighted on to the illuminated 
scratch of the sighting-point, is directed up the tunnel, where a 
thread of light is shown from the first table. With the aid of 
a telephone this light is adjusted so that its image is exactly 
coincident with the cross hairs, and the reading on the gradu- 



TL'NXELS THROUGH HARD R;)CK 99 

ated scale is noted. This is done four or five times, the aver- 
age of these readings being taken as correct, and the plane is 
clamped to that average. The instrument is then taken to the 
first table and is placed quickly and accurately over the point 
just found (by means of the sockets), and the lamp is carried 
to the observatory. After first sighting back, a second point is 
given on the second table, and so on. These points are marked 
either temporarily in the roof of the heading by a short piece 
of cord hanging down, or permanently by a brass point held by 
a small steel cylinder, 8 ins. long and 3 ins. in diameter, em- 
bedded in concrete in the rock floor, and protected by a circular 
casting, also sunk in cement concrete, holding an iron cover 
resembling that of a small manhole. From time to time the 
alinement is checked from these points by the engineers, and 
after each blast the general direction is given by the hand from 
the temporary points. To check the results of the triangula- 
tion survey, astronomical observations have been taken simul- 
taneously at each end. With regard to the levels, those given 
on the excellent Government surveys have been taken as cor- 
rect, but they have also been checked over the pass. 

Details of Tunnels. — In cross-section, tunnel No. 1 is 13 ft. 
7 ins. wide at formation level, increasing to 16 ft. 5 ins., with 
a total height of 18 ft. above rail-level, and a cross-sectional 
area of about 250 sq. ft. This large section will allow of 
small repairs being executed in the roof without interruption 
of the traffic, and will also allow of strengthening the walls by 
additional masonry on the inside. The thickness of the lining, 
never wholly absent, and the material of which it is composed, 
depend upon the pressure to be resisted, and only in the worst 
case is an invert resorted to. The side drain, to which the rock 
floor is made to slope, will be composed of half-pipes of 7 to 1 
cement concrete. The roof is constructed of radial stones. 

Tunnel No. 2, being left as a heading, is driven on that side 
nearest to No. 1, to minimize the length of the cross-headings, 
and measures 10 ft. 2 ins. wide ])y ft. 7 ins. high. Masonry 



Lof C. 



100 TUNNELING 

is used only where necessary, and in that case is so built as to 
form part of the lining of the tunnel when eventually com- 
pleted. Concrete is put in to form a foundation for the side 
wall, and a water channel. The cross-headings, connecting the 
two parallel headings, occur every 220 yds., and are placed at 
an angle of 56° to the axis of the tunnel, to avoid sharp curves 
in the contractors' railway lines. They will eventually be used 
as much as possible for refuges, chambers for storing the tools 
and equipment of the platelayers, and signal-cabins. The ref- 
uges, 6 ft. 7 ins. wide by 6 ft. 7 ins, high and 3 ft. 3 ins. deep, 
occur every 110 yards, every tenth being enlarged to 9 ft. 10 
ins. wide by 9 ft. 10 ins. deep and 10 ft. 2 ins. high, still larger 
chambers being constructed at greater intervals. 

Method of Excavation. — The work at each end of the tunnel 
is carried on quite independently, consequently, though similar 
in principle, the methods vary in detail, apart from the fact that 
different geological strata require different treatment. Broadly 
speaking, the two parallel headings, each 59 sq. ft. in section, 
are first driven by means of drilling-machines and the use of 
dynamite, this work being carried on day and night, seven days 
in the week; No. 1 heading is then enlarged to full size by 
hand-drilling and dynamite. On the Italian side, where the 
rock is hard and compact, breakups are made at intervals of 
50 yds., and a top gallery is driven in both directions, but, for 
ventilation reasons, is never allowed to get more than 1 yds. 
ahead of the breakup, which is gradually lengthened and 
widened to the required section. No timbering is required, 
except to facilitate the excavation and the construction of the 
side walls. Steel centers are employed for the arch ; they entail 
fewer supports, give more room, and are capable of being used 
over again more frequently, without damage. They consist 
of two I-beams bent to a template and riveted together at the 
crown, resting at either side on scaffolding at intervals of 6 ft. ; 
longitudinals, 12 ft. by 4 ins. by 4 ins., support the roof. Hand 
rock-drilling is carried out in the ordinary way, one man holding 



TUNNELS THROUGH HARD ROCK 



101 



the tool and 
strikino- ; 



second 




iiieasure- 
meiits of excavation 
are taken every 2 or 
3 yds., a plumb-line is 
suspended from the 
center of the roof, and 
at every half-meter 
(20 ins.) of height 
horizontal measure- 
ments are taken to 
each side. 

At the Brigue end 
a softer rock is en- 
countered, necessitat- 
ing at times heavy 
timbering in the head- 
ing, and especially in 
the final excavation 
to full size, Fig. 56. 
The bottom heading, 
6 ft. 6 in. liio^h, is 
driven in the center, 
and the heading is 
then widened to the 
full extent and tim- 
bered ; the concrete 
forming the water 
channel and the foun- 
dation for one side 
wall is put in ; the 
side walls are built to a height of 6 ft. 6 ins., and the tunnel 
is fully excavated to a further height of 6 ft. 6 ins. from the 
first staging. The side walls are then continued up for the 
second 6 ft. 6 ins., and froi.i the second floor a third height of 



7 •- ---X--- a 

Fig. 56. — Sketches Showing Sequence of Work in 
Excavating and Lining the Siniplon Tunnel. 



102 TUNNELING 

6 ft. 6 ins. is excavated and timbered. Finally the crown is 
cleared out, heavy wooden centers are put in, the arch is turned, 
and all timbers are withdrawn except the top poling-boards, 
supporting the loose rock. 

The masonry for the side walls is obtained either from the 
tunnel itself or from a neighboring quarry, and varies in char- 
acter according to the pressure ; but the face of the arch is al- 
ways of cut or artificial stones, the latter being of 7 to 1 cement 
concrete. Where the alinement heading, or the "gallery of 
direction," joins the curving portion of tunnel No. 1, the section 
is very much greater, and necessitates special timbering. 

Transport (Italian Side). — A small line of railway, 2 ft. 7 J 
ins. gauge, wdth 40-lb. rails, enters all three portals ; but since 
the construction of a wooden bridge over the Diveria, the rout^ 
througll' the "gallery of direction," across heading No. 2, to 
tunnel No. 1, is used exclusively; this railway leads to the face 
in both headings, and, where convenient, from one heading to 
the other by the cross-galleries. Different types of wagons are 
in use ; but in general they are four-wheeled, non-tipping box 
wagons, supplied with brakes and holding 2 cu. yds. of debris. 
A special type of locomotive is used, designed to pass round 
curves of 50 ft. radius, and supplied with a specially large boiler 
to avoid firing in the tunnel. 

Method of Working. — The drilling-machines employed are of 
the Brandt type, Fig. 57, and are mounted in the following 
manner: A small four-wheeled carriage supports at its center 
a beam, the shorter arm of which carries the boring incchanism 
and the longer a counterpoise ; near its center is the distributor. 
In the short arm is a clamp holding the rack-bar or butting 
column, which is a wrought-iron cylinder with a plunger con- 
stituting a ram, and is jammed by hydraulic pressure between 
the walls of the heading, thus forming a rigid support, for the 
boring-machine, and an efficient abutment against the leaction 
of the drill. This rack-bar can be rotated on its cl .mp in a 
plane parallel to the axis of the beam. Three or four -separate 



TUNNELS THROUGH HARD ROCK 103 

boring-machines caj;i be mounted on the rack-bar, and can be 
adjusted in any reasonable position. 

The boring-machine performs the double function of con- 
tinually pressing the drill into the rock by means of a hollow 
ram (1), and of imparting to the drill and ram a uniform rotary 
motion. This rotary motion is given by a twin cylinder single- 
acting hydraulic motor (^), the two pistons, of 2| ins. stroke, 
acting reciprocally as valves. The cranks are fixed at an angle 
of 90° to each other on the shaft, which carries a worm, gearing 
with a worm-wheel (§) mounted upon the shell (i?) of the 




Fig. 57. — General Details of the Brandt Rotary Drills Employed at the Simplon Tunnel. 

hollow ram (1), and this shell in turn engages the ram by a 
long feather, leaving it free to slide axially to or from the face 
of the rock. The average speed of the motor is 150 revolutions 
to 200 revolutions per minute, the maximum speed being 300 
revolutions per minute. The loss of power between the worm 
and worm-wheel is only 15 % at the most; the worm being of 
hardened steel and the wheel of gun-metal, the two surfaces in 
contact acquire a high degree of polish, resulting in little wear- 
ing or heating. Taking into consideration all other sources of 
loss, 70 % of the total power is utilized. The pressure on the 



104 TUNNELING 

drill, is exerted by a cylinder and hollow ram (J), which revolves 
about the differential piston (>S'), which is fixed to the envelope 
holding the shell (i^). This envelope is rigidly connected to 
the bed-plate of the motor, and, by means of the vertical hinge 
and pin (T), is held by the clamp (F) embracing the rack-bar. 
When water is admitted to the space in front of the differential 
piston the ram carrying the drilling-tool is thrust forward, and 
when admitted to the annular space behind the piston, the ram 
recedes, withdrawing the tool from the blast-hole. The drill 
proper is a hollow tube of tough steel 2| ins. in external diame- 
ter, armed with three or four sharp and hardened teeth, and 
makes from five to ten revolutions per minute, according to the 
nature of the rock. When the ram has reached the end of its 
stroke of 2 ft. 2i ins., the tool is quickly withdrawn from the 
hole and unscrewed from the ram ; an extension rod is then 
screwed into the tool and into the ram, and the boring is con- 
tinued, additional lengths being added as the tool grinds for- 
') ward ; each change of tool or rod takes about 15 sees, to 25 
f sees, to perform. The extension rods are forged steel tubes, 
fitted with four-threaded screws, and having the same external 
diameter as the drill. They are made in standard lengths of 
2 ft. 8 ins., 1 ft. 10 ins., and 11| ins. The total weight of the 
drilling-machine is 261 lbs., and that of the rack-bar when full 
of water is 308 lbs. The exhaust water from the two motor 
cylinders escapes through a tube in the center of the ram and 
along the bore of the extension rods and drill, thereby scouring 
away the debris and keeping the drill cool ; any superfluous 
water finds an exit through a hose below the motors and thence 
away down the heading. The distributor, already mentioned, 
supplies each boring-machine and the rack-bar with hydraulic 
pressure from the mains, with which connection is effected by 
means of flexible or articulated pipe connections, allowing free- 
dom in all directions. The area of the piston for advancing 
the tool is 151- sq. ins., which under a pressure of 1470 lbs. per 
sq. in. gives a pressure of over 10 tons on the tool, while for 



TUisNELS THKOUGH HARD KOCK 105 

withdrawing the tool 2h tons is available^ In the rock found at 
Iselle, namely, antigorio gneiss, a hole 2^ ins. in diameter and 
3 ft. 3 ins. in length ib drilled, normally, in 12 mins. to 25 mins. ; 
a daily rate of advance of 18 ft. to 19 ft. 6 ins. is made in a 
heading having a minimum cross-section of 59 sq. ft. ; the time 
taken to drill ten to twelve holes, 4 ft. 7 ins. deep, is 2J hrs. 

When the debris resulting from one operation has been 
sufficiently cleared away, a steel flooring, which is provided 
near the face to enable shoveling to be more easily done, and 
to give an even floor for the wheels of the drilling-carriage, is 
laid bare at the head of the line of rails, and the drilling- 
machines are brought up on their carriage by eight or ten 
men. When advanced sufficiently close to the face, the rack- 
bar i^ slewed round across the gallery and is wedged up against 
the rock sides ; connection is made between the distributor and 
the hydraulic main, by means of the flexible pipe, and pressure 
is supplied by a small copper tube to the rack-bar ram, thereby 
rigidly holding the machine. Next, connections are made 
between the three drilling-machines and the distributor, and in 
20 mins. from the time the machine was brought up all three 
drills are hard at work, water pouring from the holes. ^ 

The noise of the motors and grinding-tools is sufficient to 
drown all but shouts ; and where the extension rods do not fit 
tightly, small jets of water play in all directions, necessitating 
the wearing of tarpaulins by the men directing the tools. 
Lighting is done wholly by small oil-lamps, provided with a 
hook to facilitate fixing in any crack in the rock ; electricity 
will probably be used to light that portion of the tunnel which 
is completed. 

Two men are allotted to each drill, one to drive the motor, 
the other to direct and replenish the tool, one foreman and two 
men in reserve completing the gang. A small hammer is freely 
used to loosen the screw joints of the extension rods and drill. 
A hole is usually commenced by a two-edged flat-pointed tool, 
until a sufficient depth is reached to prevent the circular tool 



106 TUNNELING 

from wandering over the face of the rock, but in many instances 
the hole is commenced with a circular tool. The exhaust 
water during this period flows away by the hose underneath 
the motor. In the antigorio gneiss, ten to twelve holes are 
drilled for each attack, three to four in the center to a depth of 
3 ft. 3 ins., the remainder, disposed round the outside of the 
face, having a depth of 4 ft. 7 in. The average time taken to 
complete the holes is If hr. to 2^ hrs. Instead of pulverizing 
the rock, as do the diamond drills, it is found that the rock is 
crushed, and that headway is gained somewhat in the manner 
of a circular saw through wood. The core of rock inside the 
tool breaks up into small pieces, and can be taken out if 
necessary when the drill requires lengthening. 

The lowest holes, inclined downwards, are full of water ; 
consequently two detonators and two fuses are inserted, but 
apart from this, water has little effect on the charge. The 
fuses of the central holes are brought together and cut off 
shorter than those of the outer holes, in order that they may 
explode first to increase the effect of the outer charges. All 
portable objects, such as drills, pipe connections, tools, etc., have 
meanwhile been carried back ; the steel flooring is covered over 
with a layer of debris to prevent injury from falling rock, and 
to the end of the hydraulic main is screwed a brass plug 
pierced by five holes ; and immediately the explosions occur a 
valve is opened in the tunnel, and five jets of water play upon 
the rock, laying the dust and clearing the air. The necessity 
for this was shown on one occasion when this nozzle was 
broken by the explosion and the water had to be turned off 
immediately to avoid useless waste ; on reaching the face, the 
atmosphere was found to be so highly charged with dust and 
smoke that it was impossible to distinguish the stones at the 
feet, although a lamp had been placed on the ground; and 
despite the fact that the air tube was in full blast, the men ex- 
perienced great difficulty in breathing. A truck is now brought 
up, and four men clear a passage in front, through the heap of 



TUNNELS THROUGH HARD ROCK 107 

debris, two with picks and two with shovels, while on either 
side and behind are as many men as space will permit. The 
stone is thrown either to the sides of the heading or into the 
wagon, shoveling being greatly aided by the steel flooring, 
which, before the explosion, had been laid over the rails for 
nearly 10 yds. down the tunnel to receive the falling rock. 
These steel plates are taken up when cleared, and the wagon 
is pushed forward until the drilling-machine can be brought up 
again, leaving the remaining debris at the sides to be handled 
at leisure during the next attack. The roof and side walls are, 
of course, carefully examined with the pick, to discover and 
detach any loose or hanging rock. The times taken for each 
portion of the attack in this particular antigorio gneiss are as 
follows : Bringing up and adjustment of drills, 20 mins. ; drill- 
ing, between If hr. and 2^ hrs. ; charging and firing, 15 mins.; 
clearing away debris, 2 hrs. ; or for one whole attack, between 
4^ hrs. and 54- hrs., resulting in an advance of 3 ft. 9 in., or a 
daily advance of nearly 18 ft. 

From this it appears that the time spent in clearing away 
the debris equals that taken up in drilling, and it is in this clear- 
ing that a saving of time is likely to be effected rather than in 
the process of drilling. Many schemes have been tried, such as 
a mechanical plow for making a passage ; at Brigue, " marin- 
age," or clearing by means of powerful high-pressure water-jets, 
directed down the tunnel, was tried, but the idea is not yet 
sufiioiently developed. 

Another series of experiments has been tried at Brigue 
with regard to the utilization of liquid air as an explosive 
agent instead of dynamite ; and for this purpose a plant has been 
laid down, consisting of one ammonia-compressor, two air-com- 
pressors, and two refrigerators, furnishing j^ gallon of liquid 
air per hour at an expenditure of 17 H.P. The system used is 
that of Professor Linde, who himself directs the experiments. 
The great difficulty experienced is that of shortening the interval 
of time that must elapse between the manufacture of the 



108 TUNNELING 

cartridge and its explosion. The liquid oxygen, with which 
the cartridge, containing kieselguhr (silicious earth) and 
paraffin, is saturated, evaporates very readily, losing power 
every moment ; hence the effect of each cartridge cannot be 
guaranteed, and though it is an exceedingly powerful explosive 
when used immediately after manufacture, no practical result 
has yet been obtained. 

Power Station. — Water is abundant at either end, and there- 
fore hydraulic power is the motive force employed. On the 
Italian side, a dam 5 ft. high has been thrown across the Diveria 
at a point near the Swiss frontier, about 3 miles above the site 
of the installations. A portion of the water thus held back 
enters, through regulating doors and gratings, a masonry 
channel leading to two parallel settling tanks, each 111 ft. by 
16 ft., whence, after dropping all its sand and solid matter, the 
now pure water passes into the water-house, and, after flowing 
over a dam, through a grating and past the admission doors, 
enters a metallic conduit of 3-ft. pipes. Each of the settling 
tanks and the approach canal are provided with doors at the 
lower end leading direct to the river, through which all the 
sand and solid matter deposited can be scoured naturally by 
allowing the river-water to rush freely through. For this pur- 
pose the floor of the basins is on an average gradient of 1 in 30. 
For a similar reason the river-bed just outside the entrance to 
the approach canal is lined with wooden planks, from which 
the stones collecting behind the dam can be scoured by allow- 
ing an iron flap, hinged at the bottom, to change its position 
from the vertical to the horizontal in a gap left purposely in the 
dam, so causing a rushing torrent to sweep it clean. 

The chief levels are : 

Level of water at dam 794.00 meters above sea level. 

" in water-house 793.70 " " " " 

" at turbines 618.50 " " " " 

giving a total fall of 175.20 ms. or 570 ft., and a pressure of 
17.52 atmospheres. 



TUNNELS THROUGH HARD ROCK 



109 



The quantity of water capable of being taken from the 
Diveria in winter, Avhen the rivers which are dependent upon 
the mountain snows for their supply are at their lowest, is 
calculated to be 352 gallons per second. Thus, taking the 
fall to be diminished by friction, etc., to 440 ft., and the use- 
ful effect at 70 ^Jc, there is obtained 2,000 H.P. on the turbine 
shaft. 

The metallic conduit varies in material according to the 
pressure ; thus cast-iron pipes 3 ft. in diameter and \% in. 
thick are used up to a pressure of 2 atmospheres, from which 
point they are of wrought-iron. The cast-iron portion has of 
late caused a good deal of trouble, owing to settlement of the 
piers causing occasional bursts, consequently a masonry pier 
has been placed under each joint of this portion. The follow- 
ing table gives the thicknesses and diameters, varying with the 
pressure : 



Water 
Pressure. 


Thickness. 


Diameter. 


Weight 
PER Yard. 


Head in Feet. 


Milli- 
meters. 


Inch. 


Feet. 


Inches. 


Lbs. 


246 
311 
860 
393 
426 
476 
590 


6 

7 

8 

9 

10 

12 

16 


\ 
' 1* 


3 
3 
3 
3 
3 
3 
3 








3i 


326 

383 
481 
483 
556 
651 
977 



This pipe is supported every 30 ft. on small masonry piers, 
on the top of which is placed a block of wood hollowed out to 
receive the pipe, thus allowing any movement due to the con- 
traction and expansion of the conduit. However, to prevent 
this movement becoming excessive, the pipe is passed at 
intervals of 300 yds. to 500 yds. through a cubical block of 
masonry of 13 ft. side, strengthened by longitudinal tie-bars. 
Five bands of angle-bar riveted round tlui pipe, with their 



110 TUNNELING 

flanges embedded in the niasonrj, constitute a rigid fixed point. 
Straw mats are thrown over the pipe where it is exposed to the 
sun. The temperature of the conduit is not, however, found to 
vary greatly, since the pipe is kept full of water. To supply 
the rock-drills with water at a maximum pressure of 100 
atmospheres, or 1,470 lbs. per sq. in., a plant of four pairs of 
high-pressure pumps has been laid down, and a still larger 
addition is in course of erection. At present, two Pelton 
turbines of 250 H.P. each, running at 170 revolutions per 
minute, drive the pumps, by means of toothed gearing, at 63 
revolutions per minute. These pumps are of very simple but 
strong construction, single suction and double delivery, entail- 
ing one suction and one delivery-valve, both heavy and both of 
small lift. The larger portion of the plunger has exactly 
double the cross-sectional area of the smaller portion, so that in 
the forward stroke half of the water taken in at the last 
admission is pumped into the high-pressure mains, and at the 
same time a fresh supply of water is sucked in. During the 
backward stroke half of this new supply is pumped into 
the mains, and the remainder enters the second chamber, to 
be pumped during the next forward stroke. Thus the work 
done in the two strokes is practically the same. The pumps 
are in pairs, and are set at an angle of 90°, to insure uniform 
pressure and uniform delivery in the mains. Their size varies ; 
but at Iselle there are three pairs, with a stroke of 2 ft. 2| ins., 
and the plungers of 2 {^ in. and 1| ins. (approximately) in 
diameter, supplying 1.32 gallons per second. 

To avoid injury to the valves, the water to be pumped is 
taken from a stream up the mountain side, and is passed 
through filter screens. The high-pressure water, after passing 
an accumulator, enters the tunnel in solid drawn wrought-iron 
tubes, 3^ ins. in internal diameter, y\ in. thick, and in lengths 
of 26 ft. The diameter of these mains varies with their length, 
so as to avoid loss of pressure. With the 1,250 yds. of tunnel 
now driven 10 atmospheres are lost. 



TUNNELS THROUGH HARD ROCK 111 

At Brigue the installations are, as far as possible, identical. 
The Rhone water, however, before reaching the water-house, is 
carried from the filter basins, a distance of 2 miles, in an 
armored canal built upon the Hennebique system,* the walls 
and supporting beams, of cement concrete, being strengthened 
by internal tie-bars of steel. The concrete struts, resembling 
balks of timber at a distance, are occasionally 35 ft. high and 
1 ft. 7^ ins. square. The metallic conduit is 5 ft. in diameter, 
with a minimum flow of 176 cu. ft. per second and a total fall 
of 185 ft. In case water-power should be unavailable, three 
semi-portable steam engines, two of 80 H.P. and one of 60 H.P., 
are always kept in readiness at each end of the tunnel, and are 
geared by belts to the turbine shaft. 

Ventilation. — In tunneling, one of the most im]!)ortant prob- 
lems to be solved is that of ventilation, and it is for this reason 
that the Simplon tunnel consists of two parallel headings with 
crosscuts at intervals of 220 yds. At Brigue, a shaft 161 ft. 
deep was sunk through the overlying rock until the "gallery of 
direction " was encountered. Up this chimney the foul air is 
drawn by wood fires, the fresh air — a volume of 19,000,000 
cu. ft. per day, or 13,200 cu. ft. per minute — entering by 
heading No. 2, penetrating up to the last cross gallery, and 
returning by tunnel No. 1. The entrances of No. 1 and the 
" gallery of direction," besides those of all the intermediate 
cross galleries, are closed by doors. By this arrangement, how- 
ever, fresh air does not reach the working faces ; therefore a 
pipe, 8 ins. in diameter, is led from the fresh air in No. 2 to 
within 15 yds. of the face of each heading, and up this pipe a 
draft of air is induced by means of a jet of water, the volume 
to each face being 800 cu. ft. per minute. One single jet of 
water from the high-pressure mains, with a diameter of yV in., 
is capable of supplying over 1,000 cu. ft. of air per minute at 
the end of 160 yds. of pipe, and during the attack the men at 
the drills are in a constant l)reeze with the thermometer stand- 

* Network ot olecl rods embedded in concrete. 



112 tun:neling 

ing at 70° F. At Iselle, air is blown into the entrance of 
heading No. 2 at the rate of 14,100 cu. ft. per minute by two 
fans driven from the turbine shaft. This air travels from the 
fans along a pipe, 18 ins. in diameter, till a point 15 yds. up 
the tunnel is reached, where beyond a door the pipe narrows to 
form a nozzle lO ins. in diameter. This door is kept open to 
allow the outside air to be induced up the tunnel, as the head- 
ings are at present only 2,500 yds. long, giving a resistance of 
not quite sufficient power to cause the air to return. The fresh 
air then travels up No. 2, crossing over the top of the " gallery 
of direction," from which it is shut off by doors, to the last 
cross gallery, returning by No. 1, and finally leaving either by 
the " gallery of direction " or by No. 1. A system of cooling 
the air and driving it on by means of a large number of water- 
jets will be installed in No. 2 where that heading crosses over 
the "gallery of direction," but at present there is no need for 
it. 

The average temperature at the face is 73° F. during the 
drilling operation, 76° F. after firing the charges, and a max- 
imum of 80° F., lately attaining to 86° F. on the south side, 
with 80° F. and 85° F. before and after firing. The tempera- 
ture of the rock is taken at every 110 yds. in holes 5 ft. deep, 
and shows a gradual increase according to the depth of over- 
laying rock, to the conductivity of the rock, and to the form of 
the mountain surface. The maximum hitherto reached on the 
north side is 68° F., while on the south side, although a smaller 
distance has been traversed, it attains to 79° F., due to the 
more rapid increase in depth. Moreover, the temperature of 
the rock is observed at the permanent stations, 550 yds. from 
the entrances, in its relation to that of the tunnel and outside 
air, and though on the north side that of the rock varies almost 
as quickly as that of the tunnel air, on the south it is influenced 
very much less. 

A few statistics may be of interest with regard to the prog- 
ress of the last three months (taken from the trimestrial report 



TINNKLS THllOUGH HAKD KOCK IIB 

of January, 1900). At Brigue, where tere are three drilling- 
iiuichiiies in No. 1 and two in the parallel heading, the total 
length excavated was 995 yds. or 6,409 cu. yds. in 89 working 
days, the average cross-sectional area being 57 sq. ft. This re- 
quired 507 attacks and 3,06(5 holes, which had a total depth of 
26,600 ft., and 14,700 re-sharpenings of the drilling-tool, with 
44,000 lbs. of dynamite. 

The average time occupied in drilling was 2 hrs. 45 mms., 
while charging, firing, and clearing away the debris took 6 hrs., 
35 mins. At Brigue 648 men and 29 horses were employed at 
one time in the tunnel. At Iselle the numbers were 496 men 
and 16 horses, working in shifts of 8 hrs. Outside the tunnel, 
in the shops, forges, etc., the men work 8 hrs. to 11 hrs. per 
day, the total being 541 men at Brigue and 346 men at Iselle. 
On the ItaUan side, where the rock is very much harder, there 
were three drilling-machines in each heading ; the total length 
excavated, with a cross-sectional area of 62 sq. ft., was 960 yds. 
or 6,700 cu. yds. in 91 working days. This required 61,293 
re-sharpened tools, 758 attacks, 7,940 holes with a total depth 
of 33,000 ft,, and 56,000 lbs. of dynamite. The average time 
spent in drilling was 2 hrs. 55 mins., and in charging and clear 
ing 2 hrs. 36 mins. Thus, in the hard gneiss, to excavate 1 cu. 
yd. of rock required 8^ lbs. of dynamite, and each tool pierced 
6i ins. of rock before it required re-sharpening. 

Up to January 1, 1900, the total length of heading on the 
north side was 2,515 yds., and on the south side 1,720 yds., or 
a total of 4,235 yds. out of 21,575 yds., the full length of the 
tunnel. Allowing for unavoidable and unforeseen occurrences, 
such as strikes, war, etc., the contractors expect to complete 
tunnel No. 1 and the parallel heading by May, 1904. 



114 TUNNELING 



CHAPTER XL 

TUNNELS THROUGH HARD ROCK (Continued).— 

EXCAVATION BY HEADINGS. — ST. GOTH- 

ARD TUNNEL. — BUSK TUNNEL. 



The more common method of tunneling through hard rock 
is to begin the work by a heading, instead of by a drift. This 
heading may be of small dimensions, and the remainder of the 
section may also be removed in successive small parts, or it may 
be the full width of the section, and the enlargement of the 
section be made in one other cut. 

General Discussion. — When the tunnel is excavated by means 
of several cuts, which is the method usually employed in 
Europe, the sequence of work is as indicated by Fig. 57. 
Work is begun by driving the center top heading No 1, whose 
floor is at the level of the bottom of the roof arch, and which is 
usually excavated by the circular cut method. This heading is 
widened by removing parts No. 2 until the top part of the sec- 
tion is removed, then the roof arch is built with its feet rest- 
ing on the unexcavated rock below. The lower portion of the 
section or bench is removed by first sinking the trench No. 3, 
after which part No. 4 is taken out, and then part No. 5, and 
the side walls built. Part No. 6 for the culvert is finally 
opened. The heading is, as a rule, driven far in advance, but 
the excavation of each of the other parts follows the preceding 
one at a distance behind of about 300 ft. 

The strutting, when any is required, is usually the typical 
radial strutting of the Belgian method of tunneling. The 
masonry lining is constructed practically the same as in tunnels 
excavated by a drift. The hauling is done on a single track 
laid in the heading No. 1, which separates into double tracks 



TUNNELS THROUGH HARD ROCK 115 

where the full top section has been excavated by the removal 
of parts No. 2. These two tracks are again combined and form 
a single track along the top of part No. 5, which has been left 
wider than part No. 4 for this particular purpose. When part 
No. 3 is excavated a standard-gauge track is laid on its floor; 
and as the full section of the tunnel is completed by taking out 
parts Nos. 4 and 5, this single track is replaced by two standard- 
gauge tracks, into which it switches. Spoil is transferred from 
the narrow-gauge tracks on the upper level, to the standard- 
gauge tracks on the tunnel floor, by means of chutes, and build- 
ing material is transferred in the opposite direction by means of 
hoisting apparatus. 

When the excavation is made by a single wdde heading, and 
a single other cut for removing the bench, which is the method 
preferred by American engineers, the work begins by removing 
a top heading the full width of the section. This heading is 
usually made 7 ft. or 8 ft. high, and is excavated by the center cut 
method. The method of strutting usually employed, is to erect 
successive three- or five-segment timber arches, whose feet rest 
on the top of the bench ; when the bench is removed, posts are 
inserted under the feet of each arch. These arches are covered 
with a lagging of plank. In America it has often been the 
practice to let this strutting serve as a temporary lining, and to 
replace it only after some time, often after years, with a perma- 
nent lining of masonry. In a succeeding chapter, some of the 
methods adopted in relining timber-lined arches with masonry 
are described. The hauling is done by a narrow-gauge track 
laid on the bottom of the heading, and by either narrow or 
broad gauge tracks laid on the floor of the completed section 
below. A device called a bench carriage is often employed to 
enable the cars running on the heading tracks to dump their 
loads into the cars below, without interfering with the work on 
the Ijench front. This device consists of a wide platform 
carried on trucks, running on rails at the sides of the tunnel 
floor, so that it is level with the floor of the heading. The 



116 



TUNNELING 



front of this platform carries a hinged leaf which may be raised 
and lowered, and which forms a sort of gang-plank reaching to 
the floor of the heading. By running the heading cars out on to 
this traveling platform, they can be dumped into the cars below 
entirely clear of the work in progress on the bench front. 

For the purpose of illustrating the two methods of driving 
tunnels by a heading, which have been briefly described, the St. 
Gothard and the Busk tunnels have been selected. The St. 
Gothard tunnel is selected, as being the longest tunnel in the 
world, and because it was excavated by a number of small parts ; 
and the Busk tunnel, as being a single-track tunnel, driven by 
a heading, and bench, and having a timber lining. 

St. Gothard Tunnel. — The St. Gothard tunnel penetrates the 
Alps between Italy and France, and is 9^ miles long. It was 
constructed in 1872-82. 

Material Penetrated. — The St. Gothard tunnel was excavated 
through rock, consisting chiefly of gneiss, mica-schist, serpen- 
tine, and hornblend, the strata having an inclination of from 
45'' to 90°. At many points the rock was fissured, and disin- 
tegrated easily, and water was en- 
countered in large quantities, caus- 
ing much trouble. 

Excavation. — The sequence of 
excavation is shown by Fig. 15, 
p. 32. First the top center head- 
ing. No. 1, whose dimensions varied 
from 8.25 x 8.6 ft. to 8.5 x 9 ft., 
according to the quality of the rock, 
was driven never less than 1,000 ft. 
and sometimes over 3,000 ft. in 
advance of parts No. 2. The exca- 
vation of parts No. 2 opened up the full top section, and parts 
Nos. 3, 4, 5, 6, and 7, were removed in the order numbered. 

Strutting. — Where regular strutting was required, the con- 
struction shown in Fig. 59 was adopted. 



^ z 

4-3 5 



Tig. 58. — Diagram ShoAving Se- 
quence of Excavation in Heading 
Method of Tunneling Rock. 



TUNNELS THliOrCiH HARD KOCK 117 

3Iaso)U'i/. — The St. Gothard tunnel is lined throughout with 
masonry. After the upper portion of the section was fully 
excavated, the roof arch was built with its feet resting upon 
short planks on the top of the bench. Plank centers were used 
in constructing the arch. For the arch brick masonry was 
employed, but the side walls were built of rubble masonry. 
Shelter niches, about 3 ft. deep, were built into the side walls 
at intervals, and about every 3,000 ft. storage niches about 10 
ft. deep, and closed with a door, were constructed. The cul- 
vert was of brick masonry. 

Mechanical Installation. — Water-power was used exclusively 
in driving the St. Gothard tunnel. At the north end, the 
Reuss, and at the south end, the Tessin and the Tremola, rivers 
or torrents were dammed, and their waters conducted to tur- 
bine plants at the opposite ends of the tunnel. The power thus 
furnished by the Reuss was about 1,500 H.P., and the power 
furnished by the combined supply of the Tessin and Tremola 
was 1,220 H.P. The turbine plant at both ends at first con- 
sisted of four horizontal impulse turbines, but later, two more 
turbines were added at the south end. Each of the two sets of 
four turbines first installed drove five groups of three compres- 
sors each, and the two supplementary turbines drove two groups 
of four compressors each. The compressors were of the Colladon 
type with water injection, and four groups of three compressors 
each were capable of furnishing 1,000 cu. yds. of air compressed 
to between seven and eight atmospheres every hour, or about 
100 H.P. per hour, delivered to the drills at the front. This 
air when exhausted provided about 8,000 cu. yds. of fresh air 
per hour for ventilation. 

The compressors at each entrance discharged into a group 
of four cylindrical receivers of wrought-iron each 5.3 ft. in 
diameter by 29.5 ft. long, and having a capacity of 593 cu. ft. 
The cylinders were placed hoiizontally, the first one receiving 
the air at one end and discharginof it at the other end into the 
next cylinder, and so on. By this arrangement the air was 



118 TUNNELING 

drained of its moisture, and the discharge from the end receiver 
into the tunnel delivery pipes was not affected by the pulsations 
of the compressors. The delivery pipe decreased from 8 in. 
in diameter at the receiver to 4 ins. in diameter, and finally to 
2i. ins. in diameter, at the front. 

The drills employed were of various patterns. The first one 
employed was the Dubois & Francois " perforator," in which the 
drill-bit was fed forward by hand. This was replaced by Fer- 
roux drills having an automatic feed. Jules McKean's " perfo- 
rator " was employed at the north end of the tunnel. All of 
these drills were of the percussion type, and were mounted on 
carriages running on tracks. Their comparative efficiency was 
officially tested in drilling granitic gneiss with an operating 
air pressure of ^.b atmospheres with the following results : 

Name of Drill, Penetration Ins. per Min. 

Ferroux 1.6 

McKean 1.4 

Dubois & Francois 1.04 

Soummelier 0.85 

The heading was excavated by the circular cut method, the 
holes being driven as follows : Near the center of the heading 
three holes were first drilled, converging so as to inclose a 
pyramid with a triangular base. Around these center: holes 
from 9 to 13 others were driven parallel to the tunnel axis. 
The center holes were blasted first, and then the surrounding 
holes. From 3 to 5 hours were required to drill the two sets 
of holes, and from three to four hours were required to remove 
the blasted rock. The number of holes drilled in removing 
each of the various parts was as follows : 

Part No. 1 6 to 9 

Part No. 2 6 to 10 

Part No. 3 2 

Part No. 4 6 to 9 

Part No. 5 3 

Part No. 6 6 to 9 

Part No. 7 1 

Total for full section , 36 to^ 



TUNNELS THROUGH HARD ROCK 



119 



Hauling. — Two different systems were employed for haul- 
ing the spoil and construction material in the St. Gothard 
tunnel. To remove the spoil from parts Nos. 1 and 2 a narrow- 
gauge track was laid on the floor of the heading, and the cars 
were hauled by horses, the grade being descending from the 
fronts. These narrow-gauge care were dumped into larger 
broad-gauge cars running on the track laid on the floor of the 
completed section and hauled by compressed air locomotives 
(Fig. 59). To raise the incoming structural material from the 
broad-gauge cars to the narrow-gauge cars running on the level 
above, hoisting devices were employed. 





Method of Strutting Roof, St Gothard 
Tunnel. 



Sketch Showing Arrangement of Car 
Tracks, St. Gothard Tunnel. 



Fig. 59. 



Busk Tunnel. — The Busk tunnel, 9,094 ft. long, was built 
between Busk and Ivanhoe stations, on the Colorado Midland 
R.R. in Colorado. Fig. 60 is a transverse section of the 
tunnel ; it is for a single track, and is 15 ft. wide and 21 ft. 
high. 

Material Penetrated. — The material through which the 
tunnel was driven was a gray granite of irregular character. 
In some places the rock was found extremely hard to drill and 
blast, and stood perfectly upon exposure to the air, while in other 
places, where it seemed at first equally as hard and firm, it dis- 
integrated upon exposure, and it was found necessary to timber 



120 



TUNNELING 




the excavation. In other 
places, where no disinte- 
gration was apparent, 
the rock was full of 
seams and faults, and it 
was necessary to support 
the detached fragments 
by timbering. In a few 
places quite large cavi- 
ties were encountered, 
which were filled with 
liquid mud. In one place 
the inrush of liquid mud 
was so sudden and the 
stream so strong that 
the men barely escaped 
with their lives. 
Excavation. — The excavation was made by a heading 7 ft. 
high and the full width of the section, and by a single bench 
excavation. In driving the heading two sets of holes were 



Fig. go. 



Double Timbering in Heavy Cjround. 

• Transverse Section of Busk Tunnel Colorado 
Midland R. R., Colorado. 



TUNNELS THROUGH HARD ROCK 121 

drilled. The first set of eight holes were driven in two rows 
from top to bottom, the holes being about 2 ft. apart on the 
surface, and converging toward the center of the tunnel. 
These holes were 12 ft. deep, and the action of the blast was 
to blow out a wedge-shaped cavity in the face. The holes of 
the second set were drilled at the sides of the front and 
parallel to the sides of the section, and the blast blew out the 
remainder of the rock into the wedge-shaped center cavity. 
The method of excavating the bench was nearly the same as 
that of excavating the heading. 

Mechanical Installation. — The following machinery was 
employed in connection with the construction of the tunnel: 
at the Ivanhoe end, three 100 H. P. boilers; two 20 x 24 in. 
Ingersoll compressors, and one 20 X 24 in. Norwalk compres- 
sor; a 10 H. P. engine driving an electric-light dynamo, and a 
20 H. P. engine driving a No. 6 Baker blower, forcing fresh 
air into the tunnel through a 14-in. pipe. In the tunnel one 
No. 7 and one No. 9 Cameron pump, and a Deane duplex 
pump with a 10-in. stroke, were employed to keep the excava- 
tion clear of water, since ths grade descended uniformly from 
the Ivanhoe end, and the water followed the Avorkings. At the 
Busk end the plant consisted of three 80 H. P. boilers, two 
20 X 24-in. Ingersoll compressors, 10 H. P. and 20 H. P. en- 
gines respectively, for the electric light dynamo and the blower. 
Four 3^ in. Ingersoll eclipse drills were used in each heading, 
and two on each bencli, making six drills at each end of the 
tunnel. 

Struttitig and Lininc). — For about 78 % of its length the 
tunnel is lined with timber. The timbering consists of a five- 
segment arch for the roof, resting on a wall plate which is car- 
ried by vertical side posts. The segments of the arch, the wall 
plates, and the posts, ai'e 12 X 12-inch timbers. The roof arches 
and the j)()sts supporting the wall plates are spaced 4 ft. apart, 
center to center. Above the arches is laid a lagging of 2-inch 
longitudinal planks. The arches were set up as fast as the 



122 TUNNELING 

heading was driven, and rested upon the bench until it was 
removed and the side posts inserted. Where mud pockets 
were met the plank lagging was inserted behind the side posts 
as well as above the roof-arch ribs, and when the pressures were 
unusually great a double lining was employed. 

Progress of Work. — The rate of progress made in exca- 
vating the Busk tunnel was as follows : — 

Total time consumed in driving the heading 1,118 days 

Average daily progress for both headings 8.4 feet 

Greatest progress in one month 337 " 

Average daily progress, one month, 31 days 10.87 " 

Greatest progress in one month (28 days) at one end . . 202.5 " 

Average progress in one month (28 days) at one end . . 7.23 " 

Greatest monthly progress on bench 218 " 

Average daily progress on bench 7.79 " 

Cost of Work. — The cost of the tunnel was calculated on 
the assumption that the excavation per lineal foot was 10.19 
cu. yds., and where the section was enlarged for timbering, 
1379 cu. yds. The contractors' estimate for excavating and 
timbering the tunnel was as follows : — 

Excavation of 9,393.66 lineal feet @ $62,50 .... $587,103.73 
Enlargement for timbering 32,575 cubic yards . . . 81,437.50 

Cost of timber 81,600.00 - 

Cost of labor on timbering 2,723,000 ft. B. M. @ $12 . 32,676.00 

Total $782,817.25 

This is an average cost per lineal foot of tunnel of 183.14, 
which is very close to the average cost of single-track timber- 
lined tunnels in America, which is usually figured at $85 per 
lineal foot. 

COMPARISON OF METHODS. 

The differences between the drift and heading methods of 
excavating tunnels through rock, consist chiefly in the excava- 
tions, strutting, and hauling. When the drift method is em- 
ployed an advanced gallery is opened along the floor of the 



TUNNELS THROUGH HARD ROCK 123 

tunnel before the upper part of the section is removed, and 
when the heading method is employed the upper part of the 
section is completely excavated and lined before any part of 
the section below is excavated. When the drift method of 
driving is employed polygonal strutting is usually used, and 
longitudinal strutting is employed with the heading method of 
driving. In the drift method the hauling is done by one system 
of tracks at the same level, while in the heading method two 
systems of tracks are employed at different levels. 

It is, perhaps, impossible to state without qualification, which 
method is the better. European engineers unanimously prefer 
excavation by a drift, especially for long tunnels. An advan- 
tage that this method affords in long tunnels is, that the water 
which is usually found in large quantities under high moun- 
tains is easily collected in the drift and conveyed to the culvert, 
while in the heading method the water from the advance gallery 
before being collected into the culvert built on the floor of the 
tunnel, must pass through all the workings. This may be a 
serious inconvenience when water is found in large quantities, 
as, for instance, was the case in the St. Gothard tunnel, where 
the stream amounted to 57 gallons per second. The heading 
method has an advantage in tunneling loose rock, since it is the 
more economical in strutting. 



124 TUNNELING 



CHAPTER XII. 

REPRESENTATIVE MECHANICAL INSTALLA- 
TIONS FOR TUNNEL WORK. 



The important role played by the power plant and other 
mechanical installations in constructing tunnels through rock 
has already been mentioned. In some methods of soft-ground 
tunneling, and particularly in soft-ground subaqueous tunnel- 
ing, it is also often necessary to employ a mechanical installa- 
tion but slightly inferior in size and cost to those used in 
tunneling rock. The general character of the mechanical 
plant required for tunnel work has been described in another 
chapter. It is proposed to describe very briefly here a few 
typical individual plants of this character, which will in some 
respects give a better idea of this phase of tunnel work than 
the more general descriptions. 

Rock Tunnels. — The tunnels selected to illustrate the me- 
chanical installations employed in tunneling through rock ore : 
The Hoosac Tunnel, the Cascade Tunnel, the Niagara Falls 
Power Tunnel, the Palisades Tunnel, the Croton Aqueduct 
Tunnel, the Strickler Tunnel, in America, and the Graveholz 
Tunnel and the Sonnstein Tunnel in Europe. In addition 
there will be found in other chapters of this book a description 
of the mechanical installation at the Busk tunnel and at the 
St. Gothard and Simplon tunnels. 

Hoosac Tunnel. — The Hoosac tunnel on the Fitchburg R.R. 
in Massachusetts is 25,000 ft. long, and the longest tunnel in 
America. The material through which the tunnel was driven 
was chiefly hard granitic gneiss, conglomerate, and mica-schist 
rock. The excavation was conducted from the entrances and 



MECHANICAL INSTALLATIONS FOR TUNNEL WORK 125 

one shaft, the wide heading and single-bench method being 
employed, with the center-cut system of blasting which was 
here used for the first time. The tunnel was begun in 1854, 
and continued by hand until 1866, when the mechanical plant 
was installed. ]\Iost of the particular machines employed have 
now become obsolete, but as they were the first machines used 
for rock tunneling in America they deserve mention. The 
drills used were Burleigh percussion drills, operated by com- 
pressed air. Six of these drills were mounted on a single car- 
riage, and two carriages were used at each front. The air to 
operate these drills was supplied by air compressors operated 
by water-power at the portals and steam-power at the shaft. 
The air compressors consisted of four horizontal single-acting 
air cylinders with poppet valves and water injection. The 
compressors were designed by Mr. Thomas Deane the chief 
enofineer of the tunnel. 

Palisades Tunnel. — The Palisades tunnel was constructed to 
carry a double-track railway line through the ridge of rocks 
bordering the west bank of the Hudson River and known as 
the Palisades. It was located about opposite 116th St. in New 
York city. The material penetrated was a hard trap rock very 
full of seams in places, which caused large fragments to fall 
from the roof. The excavation was made by a single wide 
heading and bench, employing the center-cut method of blast- 
ing wdth eight center holes and 16 side holes for the 7 x 18 ft. 
heading. IngersoU-Sergeant 2^ in. drills were used, four in 
each heading and six on each bench, and 30 ft. per 10 hours 
was considered good work for one drill. 

The power-plant was situated at the west portal of the 
tunnel, and the power was transmitted by electricity and com- 
pressed air to the middle shaft and east portal workings. The 
plant consisted of eight 100 H. P. boilers, furnishing steam to 
four Rand duplex 18 X 22 in. air compressors, and an engine 
running a 30 arc light dynamo. The compressed air was car- 
ried over the ridge by pipes varying from 10 ins. to 5 ins. in 



126 TUNNELING 

(iiaineter to the shaft and to the east portal, and was used for 
operatmg the hoisting engines as well as the drills at these 
workings. Inside the tunnel, specially designed derrick cars 
were employed to handle large stones, they being also operated 
by compressed air. This car ran on a center track, while the 
mucking cars ran on side tracks, and it was employed to lift 
the bodies of the cars from the trucks, place them close to the 
front, being worked where large stone could be rolled into 
them, and return them to the trucks for removal. In addition 
to handling the car bodies the derrick was used to lift heavy 
stones. The hauling was done at first by horse-power, and 
later by dummy locomotives. 

Croton Aqueduct Tunnel. — In the construction of the Croton 
Aqueduct for the water supply of New York city, a tunnel 31 
miles long was built, running from the Croton Dam to the 
Gate House at 135th St. in New York city. The section of 
the tunnel varies in form, but is generally either a circular or a 
horse-shoe section. In all cases the section was designed to 
have a capacity for the flow of water equal to a cylinder 14 ft. 
in diameter. To drive the tunnel, 40 shafts w^ere employed. 
The material penetrated was of almost every character, from 
quicksand to granitic rock, but the bulk of the work was in 
rock of some character. The excavation in rock was conducted 
by the wide heading and bench method, employing the center- 
cut method of blasting. Four air drills, mounted on two 
double-arm columns, were employed in the heading. The 
drills for the bench work were mounted on tripods. Steam- 
power was used exclusively for operating the compressors, 
hoisting engines, ventilating fans and pumps ; but the size and 
kind of boilers used, as well as the kind and capacity of the 
machines which they operated, varied greatly, since a separate 
power-plant was employed for each shaft with a few exceptions. 
A description of the plant at one of the shafts will give an 
indication of the size and character of those at the other shafts, 
and for this purpose the plant at shaft 10 has been selected. 



MECHANICAL INSTALLATIONS FOR TUNNEL AVORK 127 

At shaft 10 steam was provided by two IngersoU boilers of 
80 H. P. each, and by a small upright boiler of 8 H. P. There 
were two 18 x 30 in. IngersoU air compressors pumping into 
two 42 in. X 10 ft. and two 42 in. X 12 ft. IngersoU receivers. 
In the excavation there were twelve 3|- in. and six 3-| in. 
IngersoU drills, four drills mounted on two double-arm columns 
being used on each heading, and the remainder mounted on 
tripods being used on the bench. Two Dickson cages operated 
by one 12 X 12 in. Dickson reversible double hoisting engine 
provided transportation for material and supplies up and down 
the shaft. A Thomson-Houston ten-light dynamo operated by 
a Lidgerwood engine provided light. Drainage was effected by 
means of two No. 9 and one No. 6 Cameron pumps. At this 
particular shaft the air exhausted from the drills gave ample 
ventilation, especially when after each blast the smoke was 
cleared away by a jet of compressed air. In other workings, 
however, where this means of ventilation was not sufficient. 
Baker blowers were generally employed. 

Strickler Tunnel. — The Strickler tunnel for the water 
supply of Colorado Springs, Col., is 6,441 ft. long with a sec- 
tion of 4 ft. X 7 ft. It penetrates the ridge connecting Pike's 
Peak and the Big Horn Mountains, at an elevation of 11,540 
ft. above sea level. The material penetrated is a coarse 
porphyritic granite and morainal debris, the portion through 
the latter material being lined. The mechanical installation 
consisted of a water power electric plant operating air com- 
pressors. The water from Buxton Creek having a fall of 
2,400 ft. was utiHzed to operate a 36 in. 220 H. P. Pelton 
water-wheel, which operated a 150 K. W. three-phase generator. 
From tliis generator a 3,500 volt current was transmitted to 
the east portal of the tunnel, where a step-down transformer 
reduced it to a 220 volt current to the motor. The transmis- 
sion line consisted of three No. 5 wires carried on cross-arm 
]X)les and provided with lightning arresters at intervals. The 
plant at the east portal of the tunnel consisted of a 75 H. P. 



128 TUNNELING 

electric motor, driving a 75 H.P. air compressor, and of small 
motors to drive a Sturtevant blower for ventilation, to run the 
blacksmith shop, and to light the tunnel, shop, and yards. 
From the compressor air was piped into the tunnel at the 
east end, and also over the mountain to the west portal work- 
ings. Two drills were used at each end, and the air was 
also used for operating derricks and other machinery. For 
removing the spoil a trolley carrier system was employed. A 
longitudinal timber was fastened to the tunnel roof, directly 
in the apex of the roof arch. This timber carried by means 
of hangers a steel bar trolley rail on which the carriages ran. 
Outside of the portal this rail formed a loop, so that the 
carriage could pass around the loop and be taken back to 
the working face. Each carriage carried a steel span of 1^ cu. 
ft. capacity, so suspended that by means of a tripping device 
it was automatically dumped when the proper point on the 
loop was reached. 

Niagara Falls Power Tunnel. — The tail-race tunnel built 
to carry away the water discharged from the turbines of the 
Niagara Falls Power Co., has a horse-shoe section 19 X 21 ft. 
and a length of 6,700 ft. It was driven through rock from 
three shafts by the center-cut method of blasting. In sink- 
ing shaft No. very little water was encountered, but at shafts 
Nos. 1 and 2 an inflow of 800 gallons and 600 gallons per 
minute, respectively, was encountered. The principal plant 
was located at shaft No. 2, and consisted of eight 100 H. P. 
boilers, three 18 x 30 in. Rand duplex air compressors, a 
Thomson-Houston electric-light plant, and a sawmill with a 
capacity of 20,000 ft. B. M. per day. The shafts were fitted 
with Otis automatic hoisting engines, with double cages at 
shafts Nos. 1 and 2, and a single cage at shaft No. 0. The 
drills used were 25 Rand drills and three Ingersoll-Sergeant 
drills. The pumping plant at shaft No. 2 consisted of four 
No. 7 and one No. 9 Cameron pumps, and that at shaft No. 2 
consisted of two No. 7 and two No. 9 Cameron pumps and 



MECHANICAL INSTALLATIONS FOR TUNNEL WOKK 129 

three Snow pumps. An auxiliary boiler plant consisting of 
two 60 H. P. boilers was located at shaft No. 1, and another, 
consisting of one 75 11. P. boiler, was located at shaft No. 0. 

Cascade Tunnel. — The Cascade tunnel was built in 1886- 
88 to carry the double tracks of the Northern Pacific Ry. 
through the Cascade ^fountains in Washington. It is 9,850 ft. 
long with a cross-section 16i ft. wide and 22 ft. high, and 
is lined with masonry. The material penetrated was a basaltic 
rock, with a dip of the strata of about 5°. The rock was 
excavated by a wide heading and one bench, using the center- 
cut system of blasting. A strutting consisting of five-segment 
timber arches carried on side posts, spaced from 2 ft. to 4 ft. 
apart, and having a roof lagging of 4 X 6 in. timbers packed 
above with cord- wood. The mechanical plant of the tunnel is 
of particular interest, because of the fact that all the machinery 
and supplies had to be hauled from 82 to 87 miles by teams, 
over a road cut through the forests covering the mountain 
slopes. This work required from Feb. 22 to July 15, 1886, to 
j)erform. In many places the grades were so steep that the 
wagons had to be hauled by block and tackle. The plant con- 
sisted of five engines, two water-wheels, five air compressors, 
eight 70 H. P. steam-boilers, four large exhaust fans, two com- 
plete electric arc-lighting plants, two fully equipped machine- 
shop outfits, 36 air drills, two locomotives, 60 dump cars, and 
two sawmill outfits, with the necessary accessories for these vari- 
ous machines. This plant was divided about equally between 
the two ends of the tunnel. The cost of the plant and of 
the work of getting it into position was 1125,000. 

Graveholz Tunnel. — The Graveholz tunnel on the Bergen 
Railway in Norway is notable as being the longest tunnel in 
northern Europe, and also as being built for a single-track 
narrow-gauge railway. This tunnel is 17,400 feet long, and is 
located at an elevation of 2,900 ft. above sea-level. Only 
about 3 % of the length of the tunnel is lined. The mechani- 
cal installation consists of a turbine plant operating the various 



130 TUNNELING 

machines. There are two turbines of 100 H. P. and 120 H. P. 
taking water from a reservoir on the mountain slope, and 
furnishing 220 H. P., which is distributed about as follows : 
Boring-machines, 60 H. P. ; ventilation, 30 to 40 H. P. ; elec- 
tric locomotives, 15 H. P. ; machine shop, 15 H. P. ; electric- 
lighting dynamo, 25 H. P. ; electric drills, the surplus, or some 
40 H. P. The boring-machines and electric drills will be 
operated by the smaller 100 H. P. turbine. 

Sonnstein TumieL — The Sonnstein tunnel in Germany is 
particularly interesting because of the exclusive use of Brandt 
rotary drills. The tunnel was driven through dolomite and 
hard limestone by means of a drift and two side galleries. The 
dimensions of the drift were 7i X 7^ ft. The power plant con- 
sisted o:^ two steam pressure pumps, one accumulator, and four 
drills. The steam-boiler plant, in addition to operating the 
pumps, also supplied power for operating a rotary pump for 
drainage and a blower for ventilation. The hydraulic pressure 
required was 75 atmospheres in the dolomite, and from 85 to 
100 atmospheres in the limestone. The drift was excavated 
with five 3i in. holes, one being placed at the center and 
driven parallel to the axis of the tunnel, and four being placed 
at the corners of a rectangle corresponding to the sides of the 
drift, and driven at an angle diverging from the center hole. 
The average depths of the holes were 4.3 ft., and the efficiency 
of the drills was 1 in. per minute. One drill was employed 
at each fi'ont, and was operated by a machinist and two helpers, 
who worked eight-hour shifts, with a blast between shifts at 
first, and later twelve-hour shifts, with a blast between shifts. 
The 24 hours of; the two shifts were divided as follows : boring 
the holes, 10.7 hours; charging the holes, 1.1 hours;" removing 
the spoil, 11.7 hours ; changing shifts, 0.5 hour. The average 
progress per day for each machine was 6.7 ft. The total cost 
oi i\ie plant was $17,450. 

aS'^. Clair River Tunnel. — The submarine double-track rail- 
way tunnel under the St. Clair River for the Grand Trunk Ry., 



MECHANICAL INSTALLATIONS FOR TUNNEL WORK 131 

is 8,500 ft. long, and was driven through clay by means of a 
shield, as described in the succeeding chapter on the shield 
system of tunneling. The mechanical plant installed for pros- 
ecuting the work was very complete. To furnish steam to the 
air compressors, pumps, electric-light engines, hoisting-engines, 
-e^ie., a steam-plant was provided on each side of the river, con- 
sisting of three 70 H. P. and four 80 H. P. Scotch portable 
boilers. The air-compressor plant at each end consisted of 
two 20 X 2-1 in. Ingersoll air compressors. To furnish light to 
the workings, two 100 candle-poAver Edison dynamos Avere in- 
stalled on the American side, and two Ball dynamos of the same 
size were installed on the Canadian side. The dynamos on 
both sides were driven by Armington & Sims engines. These 
dynamos furnished light to the tunnel workings and to the 
machine-shops and power-plant at each end. Root blowers of 
10,000 cu. ft. per minute capacity provided ventilation. The 
pumping plant consisted of one set of pumps installed for per- 
manent drainage, and another set installed for drainage during 
construction, and also to remain in place as apart of the permanent 
plant. The latter set consisted of two 500 gallon Worthington 
duplex pumps set first outside of each air lock, closing the ends 
of the river portion of the tunnel. For permanent drainage, 
a drainage shaft was sunk on the Canadian side of the river, 
and connected with a pump at the bottom of the open-cut 
approach. In this shaft were placed a vertical, direct acting, 
compound condensing pumping engine with two 19|- in. high- 
pressure and two 33} in. low-pressure cylinders of 24 in. stroke, 
connected to double-acting pumps with a capacity of 3000 
gallons per minute, and also two duplex pumps of 500 gallons 
capacity per minute. For permanent drainage on the American 
side, four Wortliiiigton pumps of 3,000 gallons' capacity were 
installed in a pum}>house set back into the slope of the open- 
cut approacli. For the permanent drainage of the tunnel 
proper two 400 gallon pumps were placed at the lowest point 
of the tunnel grade. Spoil coming from the tuimel proper was 



132 TUNNELING 

hoisted to the top of the open cut by derricks operated by two 
50 H. P. Lidgerwood hoisting-engines. The pressure pumping 
plant for supplying water to the hydraulic shield- jacks at each 
end of the tunnel consisted of duplex direct-acting engines 
with 12 in. steam cylinders and 1 in. water cylinders, supply- 
ing water at a pressure of 2000 lbs. per sq. in. 



TUNNELS THROUGH SOFT GKOUND 133 



CHAPTER XIII. 

EXCAVATING TUNNELS THROUGH SOFT 

GROUND; GENERAL DISCUSSION; THE 

BELGIAN METHOD. 



GENERAL DISCUSSION. 

It may be set down as a general truth that the excavation 
of tunnels through soft ground is the most difficult task which 
confronts the tunnel engineer. Under the general term of soft 
ground, however, a great variety of materials is included, be- 
Qfinningr with stratified soft rock and the most stable sands and 
clays, and ending with laminated clay of the worst character. 
From this it is evident that certain kinds of soft-ground 
tunneling may be less difficult than the tunneling of rock, 
and that other kinds may present almost insurmountable dif- 
ficulties. Classing both the easy and the difficult materials 
together, however, the accuracy of the statement first made 
holds good in a general way. Whatever the opinion may be 
in regard to this point, however, there is no chance for dispute 
in the statement that the difficulty of tunneling the softer and 
more treacherous clays, peats, and sands is greater than that 
of tunneling firm soils and rock ; and if we describe the methods 
which are used successfully in tunneling very unstable materials, 
no difficulty need be experienced in modifying them to handle 
stable materials. 

Characteristics of Soft-Ground Tunneling. — The principal char- 
acteristics which distinguish soft-ground tuniK^ing are, first, 
that the material is excavated without the use of explosives, 
and second, that the excavation has to be strutted practically 



134 TUNNELING 

as fast as it is completed. In treacherous soils the excavation 
also presents other characteristic phenomena; The material 
forming the walls of the excavation tends to cave and slide. 
This tendency may develop immediately upon excavation, or it 
may be of slower growth, due to weathering and other nat- 
ural causes. In either case the roof of the excavations tends 
to fall, the sides tend to cave inward and squeeze together, and 
the bottom tends to bulge or swell upward. In materials of 
very unstable character these movements exert enormous pres- 
sures upon the timbering or strutting, and in especially bad 
cases may destroy and crush the strutting completely. Out- 
side the tunnel the surface of the ground above sinks for a con- 
siderable distance on each side of the line of the tunnel. 
. Methods of Soft-Ground Tunneling. — There are a variety of 
methods of tunneling through soft ground. Some of these, 
like the quicksand method and the shield method, differ in char- 
acter entirely, while in others, like the Belgian, German, Eng- 
lish, Austrian, and Italian methods, the difference consists 
simply in the different order in which the drifts and headings 
are driven, in the difference in the number and size of these 
advance galleries, and in the different forms of strutting frame- 
work employed. In this book the shield method is considered 
individually ; but the description of the Belgian, German, Eng- 
lish, Austrian, Italian, and quicksand methods are grouped 
together in this and the three succeeding chapters to permit of 
easy comparison. 

THE BELGIAN METHOD OF TUNNELING THROUGH SOFT 

GROUND. 

The Belgian method of tunneling through soft ground was 
first employed in 1828 in excavating the Charleroy tunnel of 
the Brussels-Charleroy Canal in Belgium, and it takes its name 
from the country in which it originated. The distinctive char- 
acteristic of the method is the construction of the roof arch 



TUNNELS THllOUGH SOFT GROUND 



135 



before the side walls and invert are built. The excavation, 
therefore, begins with the driving of a top center heading 
which is enlarged until the whole of the section above the 
springing lines of the arch is opened. Various modifications 
of the method have been developed, and some of the more 
important of these will be described farther on, but we shall 
begin its consideration here by describing first the original and 
usual mode of procedure. 

Excavation. — Fig. 61 is the excavation diagram of the Bel- 
gian method of tunneling. The excavation is begun by open- 
ing the center top heading No. 1, which is carried ahead a 
greater or less distance, depending upon the nature of the soil, 
and is immediately strutted. This heading is then deepened 



3 3 

2 

5 4 5 



2 " ' 2 \ 

2 ' 

4 3 4 



Figs. 61 and 62. —Diagrams Showing Sequence of Excavations in the Belgian Method. 

by excavating part No. 2, to a depth corresponding to the 
springing lines of the roof arch. The next step is to remove 
the two side sections No. 3, by attacking them at the two fronts 
and at the sides with four gangs of excavators. The regularity 
and efficiency of the mode of procedure described consist in 
adopting such dimensions for these several parts of the section 
that each will be excavated at the same rate of speed. When 
the upper part of the section has been excavated as described, 
the roof arch is built, with its feet supported by the unexca- 
vated earth below. This portion of the section is excavated by 
taking out first the central trench No. 4 to the depth of the 
bottom of the tunnel, and then by removing the two side parts 
No. 5. As these side parts No. 5 have to support the arch, 



136 TUNNELING 

they have to be excavated in such a way as not to endanger it. 
At intervals along the central trench No. 4, transverse or side 
trenches about 2 ft. w^ide are excavated on both sides, and 
struts are inserted to support the masonry previously supported 
by the earth which has been removed. The next stej) is to 
widen these side trenches, and insert struts until all of the 
material in parts No. 5 is taken out. 

When the material penetrated is firm enough to permit, the 
plan of excavation illustrated by the diagram. Fig. 62, is substi- 
tuted for the more typical one just described. The only differ- 
ence in the two methods consists in the plan of excavating the 
upper part of the profile, which in the second method consists 
in driving first the center top heading No. 1, and then in tak- 
ing out the remainder of the section above the springing lines 
of the arch in one operation, while in the first method it is done 
in two operations. The distance ahead of the masonry to 
which the various parts can be driven varies from 10 ft. to, in 
some cases, 100 ft., being very short in treacherous ground, and 
longer the more stable the material is. 

Strutting The longitudinal method of strutting, with the 

poling-boards running transversely of the tunnel, is always 
employed in the Belgian method of tunneling. In driving the 
first center top heading, pairs of vertical posts carrying a trans- 
verse cap-piece are erected at intervals. On these cap-pieces 
are carried two longitudinal bars, which in turn support the 
saddle planks. As fast as part No. 2, Fig. 61, is excavated, 
the vertical posts are replaced by the batter posts A and B, 
Fig. 63. The excavation of parts No. 3 is begun at the top, 
the poling-boards a and b being inserted as the work pro- 
;gresses. To support the outer ends of these poling-boards, the 
longitudinals X and Y are inserted and supported by the batter 
posts C and D. In exactly the same way the poling-boards c 
iind d, the longitudinals I^and W, and the struts U and F, are 
placed in position ; and this procedure is repeated until the 
whole top part of the section is strutted, as shown by Fig. 63, 



TUNNELS THROUGH SOFT CJ ROUND 



13' 



the cross struts 2', y^ 2;, etc., being inserted to liold the radial 
struts lirmly in position. The feet of the various radial 
props rest on the sill M N. These fan-like timber structures 
are set up at intervals of from 3 ft. to 6 ft., depending upon 
the quality of the soil penetrated. 




Fig. 63. — Sketch Showing Radial Roof Strutting, Belgian Method. 

Centers. — Either plank or trussed centers may be employed 
in laying the roof arch in the Belgian method, bat the form of 
center commonly employed is a trussed center constructed as 
shown by Fig. 64. It may be said to consist of a king-post 
truss carried on top of a modified form of queen-post truss. 
The collar-beam and the tie-beam of the queen-post truss are 
spaced about 7 ft. apart, and 
the posts themselves are left far 
enough apart to allow the pas- 
sage of workmen and cars be- 
tween them. The tie beam of 
the king-post truss is clamped 
to the collar-beam of the queen- 
post truss by iron bands. On 
the rafters of the two trusses are fastened timbers, with their 
outer edges cut to the curve of the roof arch. These centers 
are set up midway between the fan-like strutting frames i)revi- 
ously described. They are usually built of square timbers. 
The tie }:)eams are usually ^^ y^ ^ in., and the struts and posts 
4x4 in. timbers. The reason for giving the larger sectional 




Fig. 64. — Sketch Showing Roof Arch 
Center, Belgian Method. 



138 TUNNELING 

dimensions to the tie beams, contrary to the usual practice in 
constructing centers, is that it has to serve as a sill for distrib- 
uting the pressure to the foundation of unexcavated soil 
which supports the center. Sometimes a sub-sill is used to 
support the center upon the soil ; and in any case wedges are 
employed to carry it, which can be removed for the purpose of 
striking the center. After the arch is completed, the centers 
may be removed immediately, or may be left in position until 
the masonry has thoroughly set. In either case the leading 
center over which the arch masonry terminates temporarily is 
left in position until the next section of the arch is built. 

Masonry. — The masonry of the roof arch, which is the first 
part built, is of necessity begun at the springing lines, and the 
first course rests on short lengths of heavy planks. These 
planks, besides giving an even surface upon which to begin the 
masonry, are essential in furnishing a bearing to the struts 
inserted to support the arch while the earth below them, part 
No. 5, Fig. 61, is being excavated. As the arch masonry 
progresses from the springing lines upward, the radial posts 
of the strutting are removed, and replaced by short struts rest- 
ing on the lagging of the centers, which support the crown 
bars or longitudinals until the masonry is in place, when they 
and the poling-boards are removed, and the space between the 
arch masonry and walls of the excavation is filled with stone 
or well-rammed earth. 

Considering now the side wall masonry, it will be re- 
membered that in excavating the part No. 5, Fig. 61, of the 
section, frequent side trenches were excavated, and struts 
inserted to take the weight of the masonry. These struts are 
inserted on a batter, with their feet near the center of the 
tunnel floor, so that the side wall masonry may be carried up 
behind them to a height as near as possible to the springing 
lines of the arch. When this is done the struts are removed, 
and the space remaining between the top of the partly fin- 
ished side wall and the arch is filled in. This leaves the arch 



TUNNELS THKOUGH SOFT GKOUND 



139 



supported by alternate lengths or pillars of unexcavated earth 
and completed side wall. The next step is to remove the 
remaining sections of earth between the sections of side wall, 
and fill in the space with masonry. 
Fig. 65 is a cross-section, showing 
the masonry completed for one-half 
and the incUned props in position 
for the other half; and Fig. 67 is 
a longitudinal section showing the 
pillars of unexcavated earth be- 
tween the consecutive sets of in- 
clined struts and several other 
details of the lining, strutting, and 
excavating work. 

The invert masonry is built after 
the side walls are completed. This 
is regarded as a defect of this method of tunneling, since the 
lateral pressures may squeeze the side walls together and dis- 
tort the arch before, the invert is in place to brace them apart. 




Fig. 65. — Sketch Showing Method of 
Underpining Roof Arch with the 
Side Wall Masonry. 




To prevent as much as possible 
the distortion of the arch after 
the centers are removed, it is 
considered good practice to 
shore the masonry with hori- 
zontal beams having their ends 
abutting against plank, as shown by Fig. 65. These hori- 
zontal beams should be placed at close intervals, and be 
supported at intermediate points by vertical posts, as sho^vn 



Fl<i. M.— I.(>n;,'ituiliiial Section Showing 
Construction by the Belgian Method. 



140 TUNNELING 

by the illustration. Since the roof arch rests for some time 
supported directly by the unexcavated earth below, settle- 
ment is liable, particularly in working through soft ground. 
This fact may not be very important so long as the settle- 
ment is uniform, and is not enough to encroach on the space 
necessary for the safe passage of travel. To prevent the 
latter possibility the centers are placed from 9 ins. to 15 ins. 
higher than their true positions, depending upon the nature of 
the soil, so that considerable settlement is possible without any 
danger of the necessary cross-section being infringed upon. 
In conclusion it may be noted that the lining may be con- 
structed in a series of consecutive rings, or as a single cylin- 
drical mass. 

Hauling. — Since in this method of tunneling the upper part 
of the section is excavated and lined before the excavation of 
the lower part is begun, the upper portion is always more ad- 
vanced than the lower. To carry away the earth excavated at 
the front, therefore, an elevation has to be surmounted ; and 
this is usually done by constructing an inclined plane rising 
from the floor of the tunnel to the floor of the heading, as shown 
by Fig. 66, This inclined plane has, of course, to be moved ahead 
as the work advances, and to permit of this movement with as 
little interruption of the other work as possible, two planes are 
employed. One is erected at the right-hand side of the section, 
and serves to carry the traffic while the left-hand side' of the 
lower section is being removed some distance ahead and the 
other plane is being erected. The inclination given to these 
planes depends upon the size of the loads to be hauled, but they 
should always have as slight a grade as practicable. Narrow- 
gauge tracks are laid on these planea and along the floor of the 
upper part of the section passing through the center opening 
mentioned before as being left in the centers and strutting. 

In excavating the top center heading there is, of course, an- 
other rise to its floor from the floor of the upper part of the 
section. Where, as is usually the case in soft soils, this top 



TUNNELS THROUGH SOFT GROUND 141 

heading is not driven very far in advance, the earth from the 
front is usually conveyed to the rear in wheelbarrows, and 
dumped into the cars s binding on the tracks below. In firm 
soils, where the heading is driven too far in advance to make 
this method of conveyance inadequate, tracks are also laid on 
the floor of the heading, and an inclined plane is built connect- 
ing it with the tracks on the next level below. In place of 
these inclined planes, and also in place of those between the floor 
of the tunnel and the level above, some form of hoisting device 
is sometimes employed to lift the cars from one level to the 
other. There are some advantages to this method in point of 
economy, but the hoisting-machines are not easily worked in 
the darkness, and accidents are likely to occur. 

In the advanced top heading and in the upper part of the 
section narrow-gauge tracks are necessarily employed, and these 
may be continued along the floor of the finished section, or the 
permanent broad-gauge railway tracks may be laid as fast as 
the full section is completed. In the former case the perma- 
nent tracks are not laid until the entire tunnel is practically 
completed ; and in the latter case, unless a third rail is laid, the 
loads have to be transshipped from the broad- to the narrow- 
gauge tracks or vice versa. It is the more general practice to 
use a third rail rather than to transship every load. 

Modifications. — Considering the extent to which the Belgian 
method of tunneling has been employed, it is not surprising 
that many modifications of the standard mode of procedure 
have been developed. The modification which differs most 
from the standard form is, perhaps, that adopted in excavating 
the Roosebeck tunnel in Germany. This method preserves the 
principal characteristic of the Belgian method, which is the 
construction of the upper part of the section first ; but instead 
of building the side walls from the bottom upward, they are 
built in small sections from the top downward. The excavation 
begins by driving the center to[) heading No. 1, Fig. 67, whose 
floor is at the level of the springing lines of the roof arch, and 



142 



TUNNELING 



/ 2 




I 


Z \ 


A 


3 


6 


5 


6 


7 



Fig. 67. — Diagram Show- 
ing Sequence of Excava- 
tion in Modified Belgian 
Method. 



then the two side parts No. 2 are excavated, opening up the 
entire upper portion of the section in which the roof arch is 
built, as in the regular Belgian method. The next step is to 
excavate part No. 3, shoring up the arch 
at frequent intervals. Between these sets 
of shoring the side walls are built, resting 
on planks on the floor of part No. 3, and 
then the sets of shores are removed and re- 
placed by masonry. Next part No. 4 is 
excavated, shored, and filled with masonry 
as was part No. 3. In exactly the same 
way parts 5, 6, 7, and 8 are constructed 
in the order numbered. To prevent the 
distortion of the arch during the side-wall 
construction it is braced by horizontal struts, as deseribed 
above in Fig. 65. 

Advantages. — The advantages of the Belgian method of 
tunneling may be summarized as follows : (1) The excavation 
progresses simultaneously at several points without the differ- 
ent gangs of excavators interfering with each other, thus secur- 
ing rapidity and efficiency of work; (2) the excavation is done 
by driving a number of drifts or parts of small section, which 
are immediately strutted, thus causing the minimum disturb- 
ance of the surrounding material ; (3) the roof of the tunnel, 
which is the part of the lining exposed to the greatest pressures, 
is built first. 

Disadvantages. — The disadvantages of the Belgian method 
of tunneling may be summarized as follows : (1) The roof arch 
which rests at first on compressible soil is liable to sink ; (2) 
before the invert is built there is danger of the arch and side 
walls being distorted or sliding under the lateral pressures ; (3) 
the masonry of the side walls has to be underpinned to the arch 
masonry. 

Accidents and Repairs. — One of the most frequent accidents 
in the Belgian method of tunneling is the sinking of the roof 



TUNNELS THROUGH SOFT GROUND 



x43 



arch owing to its unstable foundation on the unexcavated soil 
of the lower portion of the section. The amount of settlement 
may vary from a few inches in firm soil to over 2 ft. in loose 
soils. To counteract the effect of this settlement it is the gene- 
ral practice to build the arch some inches higher than its nor- 
mal position. When the settlement is great enough to infringe 
seriously upon the tunnel section, repairs have to be made ; and 
the only way of accomplishing them is to demolish the arch and 
rebuild it from the side walls. It is usually considered best not 
to demohsh the arch until the invert has been placed, so that 

no further disturbance is likely once 
the lining is completed anew. 

The rotation of the arch about its 
keystone, or the openmg of the arch at 
the crown, by the squeezing inward of 
the haunches by the lateral pressures, 
is another characteristic accident. Fig. 
68 shows the nature of the distortion 
produced ; the segments of the arch 
move toward each other by revolving 
on the intradosal edges of the keystone, 
which are broken away and crushed together with the operation, 
while the extradosal edges are opened. It is to prevent this 
occurrence that the horizontal struts shown in Fig. 65 are em- 
ployed. The manner of repairing this accident differs, depend- 
ing upon the extent of the injury. When the intradosal edges 
of the keystone are but slightly crushed, the repairing is done 
as directed by Fig. 69. When the keystone is completely 
crushed, however, the indications are that the material of the 
keystone, usually brick, is not strong enough to resist the 
pressures coming upon it, and it is advisable to substitute a 
stronger material in the repairs, and a stone keystone is con- 
structed as shown })y Fig. 70. The middle stone of this key- 
stone extends througli the depth of the arch ring, and the two 
side stones only half-way through, their purpose being merely 




Fig. 68. — Sketch Showing 
Failure of Roof Arch by 
Opening at CrowTi. 



144 



TUNNELjjSG 



to resist the crushing forces which are greatest at the intrados. 
Sometimes, when the pressures are unsymmetrical, the arch 
ring breaks at the haunches as well as the crown, as shown by 




Figs. 69 to 71. — Sketclies Showing Methods of Repairing Roof Arch Failures. 

Fig. 71, which also indicates the mode of repairing. This 
consists in demolishing the original arch, and rebuilding it 
with stone voussoirs inserted in place of the brick in which the 
rupture occurred^ 



GERMAN METHOD 



145 



CHAPTER XIV. 

THE GERMAN METHOD OF EXCAVATING 

TUNNELS THROUGH SOFT GROUND; 

BALTIMORE BELT LINE TUNNEL. 



The German method of tunneling was first used in 1803 
in constructing the St. Quentin Canal. In 1837 the Konigs- 
dorf tunnel of the Cologne and Aix la Chapelle R.R. was 
excavated by the same method. The success of the method in 
these two difficult pieces of soft-ground tunneling led to its 
extensive adoption throughout Germany, and for this reason 
it gradually came to be designated as the German method. 
Briefly explained the method consists in excavating first an 
annular gallery in which the side walls and roof arch are built 
complete before taking out the center core and building the 
invert. 

Excavation. — The excavation of tunnels by the German 
method is begun either by driving two bottom side drifts or 
by driving a center top heading. Fig. 73 shows the mode of 



/ ^ 




3 




4 \ 


2 


5 


Z 






1 




1 



/ 5 


4 


5 \ 


3 


6 


3 


e 


2 


I 


1 



Figs. 72 and 73. — Diagrams Showing Sequence of Excavation in German Method 

of Tunneling. 

procedure when bottom side drifts are used to start the work. 
The two side drifts No. 1 are made from 7 ft. to 8 ft. wide, 
and about one-third the total height of the full section ; the 



146 



TUNNELING 



width of each heading has to be sufficient for the construction 
of the masonry and strutting, and for the passage of narrow 
.spoil cars alongside them. These drifts are increased in height 
to the springing line of the arch by taking out the two drifts 
^o. 2. Next the top center heading No. 3 is driven, and 
iinally the two haunch headings No. 4 are excavated. The 
center core No. 5 is utilized to support the strutting until 
the side walls and roof arch are completed, when it is broken 
down and removed. In case of very loose material, where the 
first side drifts cannot be carried as high as one-third the 
height of the section, it is the common practice to make them 
about one-fourth the height, and to take out the side portions 
of the annular gallery in three parts, as 
shown by Fig. 73. 

The top center heading plan of com- 
mencing the excavation is usually em- 
ployed in firm materials or when a vein 
of water is encountered in the upper part 
of the section. In the latter contingency 
a small bottom drift A, Fig. 74, is first 
driven to serve as a drain ; but in any 
case the excavation proper of the tunnel 
consists in first driving the center top 
heading No. 1, and then by working both 
Avays along the profile parts, Nos. 2, 3, 4, and 5 are removed. 
Part No. 6 is left to support the strutting until the side walls 
^nd roof arch are built, when it is also excavated. 

Strutting. — When the excavation is begun by bottom side 
drifts these drifts are strutted by erecting vertical posts close 
•against the sides of the drift and placing a cap-piece trans- 
versely across the roof of the drift. The side posts are 
usually supported by sills placed across the bottom of the drift. 
These frameworks of posts, cap, and sill are erected at short 
intervals, and the roof, and, if necessary, the sides of the drift 
l)etween them, are sustained by means of longitudinal poling- 



f^ 


I 


>^ 


4 


6 


4 



FiG. 74.— Diagram Show- 
ing Sequence of Excava- 
tions in Water Bearing 
Material, German 
Method. 



GERMAN METHOD 



147 



boards extending from one frame to the next. The cap-pieces 
of the strutting for the bottom drifts serve as sills for the 
exactly simiUir strutting of the heading next above. To sup- 
port the additional weight, and to allow the construction of the 
side walls, the strutting of the bottom drifts is strengthened by 
inserting an intermediate post between the original side posts 
of each frame. These intermediate posts are not inserted at 
the center of the frames or bents, but close to the wall masonry 
line as shown by Fig. 75. This eccentric position of the post 




Fig. 75. — Sketch SboAving Work of Ex- 
cavating and Timbering Drifts and 
Headings. 




Fig. 76. — Sketch Showing Method of 
Koof Strutting. 



avoids any interference with the hauling, and also allows the 
removal of the adjacent side post when the masonry is 
constructed. 

Two methods of strutting the soffit of the excavation are 
employed, one being a modification of the longitudinal system 
employed in the English method of tunneling described in a 
succeeding chapter, and the other a modification of tlie Belgian 
system previously described. Fig. 76 shows the method of 
employing the radial strutting of the Belgian system. At the 
beginning the center top heading is strutted with rectangular 
bants such as are employed for strutting the drifts. As this 
heading is enlarged by taking out the haunch sections, radial 
posts are inserted, as shown by Fig. 76, which also indicates 



148 



TUNNELING 



the method of strutting the side trenches when the excavation 
is carried downward from the center top heading instead of 
upward from bottom side drifts. 

Masonry. — Whatever plan of excavation or strutting is 
employed, the construction of the masonry lining in the German 
method of tunneling begins at the foundations of the side walls 
and is carried upward to the roof arch. The invert, if one is 
required, is built after the center core of earth is removed. 

Centering. — Tunnel centers are generally employed in the 
German method of tunneling, a common construction being 

shown by Fig. 77. It is essen- 
tially a queen-post truss, the tie 
beam of which rests on a transverse 
sill as showai by the illustration. 
The transverse sill is supported 
along its central portion by the 
unexcavated center core of earth, 
and at its ends either directly on 
the vertical posts or on longitudi- 
nal beams resting on these posts. 
The diagonal members of the 
queen-post truss form the bottom 
chords of small king-post trusses 
which are employed to build out the exterior member of the 
center to a closer approximation to the curve of the arch. 

Hauling. — When the bottom side drift plan of excavation 
is employed, the spoil from the front of the drift is removed in 
narrow-gauge cars running on a track laid as close as practicable 
to the center core. These same cars are also employed to take 
the spoil from the drifts above, through hol-es left in the ceiling 
strutting of the bottom drifts. The spoil from the sofBt sec- 
tions may be removed by the same car lines used in excavating 
the drifts, or a narrow-gauge track may be laid on the top of the 
center core for this special purpose. In the latter case the soffit 
tracks are usually connected by means of inclined planes witl) 




Fig. 77. — Sketch Showing Koof Arch 
Centers and Arch Construction. 



GERMAN METHOD 149 

the tracks on the bottoms of the side drifts. Generally, how- 
ever, the separate soffit car line is not used unless the material 
is of such a firm character that the headings and drifts can be 
carried a great distance ahead of the masonry work. With the 
center top heading plan of beginning the excavation, the car 
track has, of course, to be laid on the top of the center core. 
The center core itself is removed by means of car tracks along 
the floor of the completed tunnel. 

Advantages and Disadvantages. — Like the Belgian method 
of tunneling, the German method has its advantages and dis- 
advantages. Since the excavation consists at first of a narrow 
annular gallery only, the equilibrium of the earth is not greatly 
disturbed, and the strutting does not need to be so heavy as in 
methods where the opening is much larger. The undisturbed 
center core also furnishes an excellent support for the strutting, 
and for the centers upon which the roof arches are built. 
Another important advantage of the method is that the con- 
struction of the masonry lining is begun logically at the bottom, 
and progresses upward, and a more homogeneous and stable 
construction is possible. The great disadvantage of the method 
is the small space in which the hauling has to be done. The 
spoil cars practically fill the narrow drifts in passing to and from 
the front, and interfere greatly with the work of the carpenters 
and masons. Another objection to the method is that the 
invert is the very last portion of the lining to be built. This 
may not be a serious objection in reasonably compact and stable 
materials, but in very loose soils there is always the danger of 
the side walls being squeezed together before the invert masonry 
is in position to hold them apart. Altogether tlie difficulties 
are of a character which tend to increase the expense of the 
method, and this is the reason why to-day it is seldom used 
even in the country where it was first developed, and for some 
time extensively employed. For repairing accidents, such as 
the caving in of completed tunnels, the German method of tun- 
neling is frequently used, because of tlie ease with which the 



150 TUNNELING 

timbering is accomplished. In such cases the cost of the 
method used cuts a small figure, so long as it is safe and 
expeditious. 

BALTIMORE BELT LINE TUNNEL. 

The Baltimore Belt Ry. Co. was organized in 1890 by 
officials of the Baltimore & Ohio, and Western Maryland rail- 
ways, and Baltimore Capitalists, to build 7 miles of double track 
railway, mostly within the city limits of Baltimore. This rail- 
way was partly open cut and embankment, and partly tunnel, 
and its object was to afford the companies named facilities for 
reaching the center of the city with their passengers and freight. 
To carry out the work the Maryland Construction Co. was 
organized by the parties interested, and in September, 1890, this 
company let the contract for construction to Rayan & McDon- 
ald of Baltimore, Md. The chief difficulties of the work cen- 
tered in the construction of the Howard-street tunnel, 8,350 ft. 
long, running underneath the principal business section of 
the city. 

Material Penetrated. — The soil penetrated by the tunnel 
was of almost all kinds and consistencies, but was chiefly sand 
of varying degrees of fineness penetrated by seams of loam, 
clay, and gravel. Some of the clay was so hard and tough that 
it could not be removed except by blasting. Rock was also 
found in a few places. For the most part, however, the work 
was through soft ground, furnishing more or less water, which 
necessitated unusual precautions to avoid the settling of the 
street, and consequent damage to the buildings along the line. 
A large quantity of water was encountered. Generally this 
water could be removed by drainage and pumps, and the earth 
be prevented from washing in by packing the space between the 
timbering with hay or other materials. At points where the 
inflow was greatest, and the earth was washed in despite 
the hay packing, the method was adopted of driving 6-in. per- 



GERMAN METHOD 151 

f orated pipes into the sides of the excavation, and forcing- 
cement grout through them into the soil to solidify it. These- 
pipes penetrated the ground about 10 ft., and the methods 
proved very efficient in preventing the inflow of water. 

Excavation. — The excavation was carried out according to» 
the German method of tunneling. Bottom side drifts were 
first driven, and then heightened to the springing line of the- 
reof arch. Next a center top heading was driven, and the- 
haunch sections taken out. The object of beginning the exca- 
vations by bottom side drifts, was to drain the soil of the upper 
part of the section. The center core was removed after the 
side walls and roof arch were completed, its removal being- 
kept from 50 ft. to 75 ft. to the rear of the advanced heading. 
The dimensions of the side drifts proper were about 8x8 ft.^ 
but they were often carried down much below the floor level 
to secure a solid foundation bed for the side walls. 

Strutting. — The side drifts were strutted by means of 
frames composed of two batter posts resting on boards, and 
having a cap-piece extending transversely across the roof 
of the drift. These frames were spaced about 4 ft. apart. 
The excavation was advanced in the usual way by driving 
poling-boards at the top and sides, with a slight outward and 
upward inclination, so that the next frame could be easily 
inserted with additional space enough between it and the 
sheeting to permit the next set of poling-boards to be inserted. 
These poling-boards were driven as close together as practicable 
so as to prevent as much as possible the inflow of water and 
earth. 

The center toj) heading was strutted in the same manner as? 
were the side drifts. The arrangement of the strutting em- 
ployed in enlarging the center top heading is shown clearly by 
Fig. 78, which also shows the manner of strutting the side 
drifts and face of the excavation, and of building the masonry. 

Centers Both wood and iron centers were employed in 

building the roof arch. The timber centering was constructed 



152 



TUNNELING 



of square timbers, as shown by Fig. 79. This construction of 
the iron centers is shown by Fig. 80. Each of the iron centers 
consisted of two 6 x 6 in. angles butted together, and bent into 
the form of an arch rib. Six of these ribs were set up 4 ft. 
apart. They were made of two half I'ibs butted together at the 
crown, and were held erect and the proper distance apart by 




Fig. 78. — Sketct. Showing Method of Excavating and Strutting Baltimore Belt 
Line Tunnel. 

spacing rods. The rearmost rib was held fast to the completed 
arch masonry, and in turn supported the forward ribs while the 
lagging was being placed. 

Masonry. — The side walls of the lining were built first in 
the bottom side drifts, as shown by Fig. 78. They were gen- 
erally placed on a foundation of concrete, from 1 ft. to 2 ft. 
thick. As a rule the side walls were not built more than 20 
ft. in advance of the arch, but occasionally this distance was 
increased to as much as 90 ft. The roof arch consisted ordina- 



GERMAN METHOD 



153 



rily of five rings of brick, but at some places in especially un- 
stable soil eight rings of brick were employed. The arch was 
built in concentric sections about 18 ft. in length. All the 
timber of the strutting above the arch and outside of the side 
walls was left in place, and the voids were filled with rubble 
masonry laid in cement mortar. It required about 125 mason 




Fig. 79. — Roof Arch Con.structiou with Timber Centers, Baltimore Belt Line Tunnel. 



hours to Ijuild an 18-ft. arch section. Figs. 79 and 80 show 
various details of the masonry arch work. 

Owing to the very unstable character of the soil, consider- 
able difficulty was experienced in building the masonry invei-t. 
The process adopted was as follows; Two parallel 12 X 12 in. 
timbers were first placed transversely across the tunnel, abutting 
against longitudinal timbers or wedges resting against the side 
walls. Short sheet piles were then driven into the tunnel 



154 



tu:nneling 



bottom outside of these timbers, forming an in closure similar 
to a cofferdam, from which the earth could be excavated with- 
out disturbing the surrounding ground. The earth being 
excavated, a layer of concrete 8 ins. thick was placed, and the 
brick masonry invert constructed on it. In less stable ground 
each of the above described cofferdams was subdivided by 
transverse timbers and sheet piling into three smaller , coffer- 
dams. Here the masonry of the middle section was first con- 
structed, and then the side sections built. Where the ground 




Fig. 80. — Roof Arch Construction with Iron Centers, Baltimore Belt Line Tunnel. 



was worst, still more care was necessary, and the bottom had 
to be covered with a sheeting of 1^-in. plank held down by 
struts abutting against the large transverse timbers. The 
invert masonry was constructed on this sheeting. Refuge 
niches 9 ft. high, 3 ft. wide, and 15 ins. deep were built in the 
side walls. 

Accidents. — In this tunnel, owing to the quick striking of 
the centers, it was found that the masonry lining flattened at 
the crown and bulged at the sides. This was attributed to the 
insufficient time allowed for the mortar to set in the rubble 



GERMAN METHOD 155 

filling. Earth packing was tried, but gave still worse results. 
Finally dry rubble filling was adopted, with satisfactory results. 
There was necessarily some sinking of the surface. This re- 
sulted partly from the necessity of changing and removing of 
the timbei-s, and from the compression and springing of the 
timbers under the great pressures. The crown of the arch also 
settled from 2 ins. to 6 ms., due to the compression of the 
mortar in the joints. The maximum sinking of the surface of 
the street over the tunnel was about 18 ins. ; it usually ran 
from 1 to 12 ins. Some damage was done to the water and gas 
mains. This damage was not usually serious, but it of course 
necessitated immediate repairs, and in some instances it was 
found best to reconstruct the mains for some distance. At one 
point along the tunnel where very treacherous material was 
found, the surface settlement caused the collapse of an adjacent 
building, and necessitated its reconstruction. 



156 TUNNELING 



CHAPTER XV. 

THE FULL SECTION METHOD OF TUNNELING 
ENGLISH METHOD; AUSTRIAN METHOD. 



ENGLISH METHOD. 

The English method of tunneling through soft ground, as 
its name implies, originated in England, where, owing to the 
general prevalence of comparatively firm chalks, clays, shales, 
and sandstones, it has gained unusual popularity. The dis- 
tinctive characteristics of the method are the excavation of the 
full section of the tunnel at once, the use of longitudinal strut- 
ting, and the alternate execution of the masonry work and 
excavation. In America the method is generally designated as 
the longitudinal bar method, owing to the mode of strutting, 
which has gained particular favor in America, and is commonly 
employed here even when the mode of excavation is distinc- 
tively German or Belgian in other respects. 

Excavation. — Although, as stated above, the distinctive 
characteristic of the English method is the excavation of the 
full section at once, the digging is usually started by driving 
a small heading or drift to locate and establish the axis of the 
tunnel, and to facilitate drainage in wet ground. These ad- 
vance galleries may be driven either in the upper or in the 
lower part of the section, as the local conditions and choice 
of the engineer dictate. Whether the advance gallery is located 
at the top or at the bottom of the section makes no difference in 
the mode of enlarging the profile. This work always begins 
at the upper part of the section. A center top heading is 
driven and strutted by erecting posts carrying longitudinal bars 
supporting transverse poling-boards. This heading is imme- 




THE FULL SECTION METHOD 157 

diately widened by digging away the earth at each side, and by 
strutting the opening by temporary posts resting on blocking, 
and carrying longitudinal bars supporting poling-boards. This 
process of widening is continued in this manner until the full 
roof section, No. 1, Fig. 81, is opened, when a heavy transverse 
sill is laid, and permanent struts are 
erected from it to the longitudinal bars, 
the temporary posts and blocking being 
removed. The excavation of part No. 2 
then begins by opening a center trench 
and widening it on each side, temporary 
posts being erected to support the sill 
above. As soon as part No. 2 is fully ex- 
cavated, a second transverse sill is placed ^^^' si.— Diagram show- 

^ ing Sequence of Excava- 

below the first, and struts are placed tion in English Method 
between them. The excavation of part ^ "^"^ "^^' 
No. 3 is carried out in exactly the same manner as was part 
No. 2. The lengths of the various sections, Nos. 1, 2, and 3, 
generally run from 12 ft. to 20 ft., depending upon the 
character of the soil. 

Strutting The strutting in the English method of tunnel- 
ing consists of a transverse framework set close to the face of 
the excavation, which supports one end of the longitudinal 
crown bars, the other ends of which rest on the completed 
lining. The transverse framework is composed of three hori- 
zontal sills arranged and supported as shown by Fig. 82. The 
bottom sill A is carried by vertical posts resting on blocking on 
the floor of the excavation. From the bottom sill vertical 
struts rise to support the middle sill B. The top sill, or miners' 
sill C, is carried by vertical posts or struts rising from the 
middle sill B. The vertical struts are usually round timbers 
from 6 ins. to 8 ins. in diam.eter ; and the sills are square tim- 
])ers of sufficient section to carry the vertical loads, and gener- 
ally made up of two posts scarf-jointed and butted to permit 
them to be more easily handled. In firm soils the struts be- 



158 



TUNNELING 



tween the sills are all set vertically, but those at the extreme 
sides of the roof section are inclined. In loose soils, however, 
where the sides of the excavation must be shored, the V- 
bracing shown by Fig. 82 is employed between one or more 
pairs of sills as the conditions necessitate. The manner of 
holding the transverse framework upright is explained quite 
clearly by Fig. 83 ; inclined props extending from the com- 
pleted masonry to the sills of the framework being employed. 
Two props are used to each sill. Sometimes, in addition to the 




Figs. 82 and 83. — Sketches Showing Construction of Strutting, English Method. 

props shown, another nearly horizontal prop extends from the 
crown of the arch masonry to the middle piece of the strutting. 
Referring to Fig. 83, it will be observed that the longitudinal 
crown bars are above the extrados of the roof arch. When, 
therefore, the lining masonry has been completed close up to 
the transverse framework, the latter is removed, leaving the 
crown bars resting on the arch masonry ; and excavation, which 
has been stopped while the masonry was being laid, is continued 
for another 12 ft. to 20 ft, and the transverse framework is 
erected at the face, and braced or propped against the completed 
lining as shown by Fig. 83. The next step is to place the 



THE FULL SECTION METHOD 



159 



crown bars, and this is done by pulling them ahead from their 
original position over the masonry of the completed section of 
the roof arch. It will be understood that the crown bars are 
not pulled ahead their full length at one operation, but are 
advanced by successive short movements as the excavation 
progresses, their outer ends being suj^ported by temporary 
posts until the transverse framework is built at the face of the 
excavation. 

Centers. — Two standard forms of centers are employed in 
the English method of tunneling, as shown by Figs. 84 and 85. 
Both consist of an outer portion, constructed much like a 
typical plank center, which is strengthened against distortion 
by an interior truss framework. The elemental members of 




Figs. 84 and 85.— Sketches of Typical Timber Roof-Arch Centers, English Method. 

this truss framework take the form of a queen-post truss, as is 
shown more particularly by Fig. 84. In Fig. 85 the queen- 
post truss construction is less easily distinguished, owdng to 
the cutting of the bottom tie-beam and other modifications, but 
it can still be observed. The possibility of cutting the tie-beam 
as shown in Fig. 85, without danger, is due to the fact that 
the lateral pressures on the haunches of the center counteract 
the tendency of the center to flatten under load, which is 
usually counteracted by the tie-beam alone. The object of 
cutting the tie-beam is to afford room for the props running 
from the completed masonry to the transverse framework of 
the strutting as shown by Fig. 83. 

Generally four or five centers are used for each length of 
arch built. They are set up so that the tie-beams rest on 



160 TUNNELING 

double opposite wedges carried by a transverse beam below. 
This transverse beam in turn rests on another transverse beam 
which is supported by posts carried on blocking on the invert 
masonry. It is usually made with a butted joint at the middle 
to permit its removal, since it is so long that the masonry has 
to be built around its extreme ends. The lagging is of the 
usual form, and rests on the exterior edges of the curved upper 
member of the centers. 

Masonry. — In the English method of tunneling, the masonry 
begins with the construction of the invert, and proceeds to the 
crown of the arch. The lining is built in lengths, or successive 
rings, corresponding to the length of excavation, which, as pre- 
viously stated, is from 12 ft. to 20 ft. Each ring or length of 
lining terminates close to the transverse strutting frame erected 
at the face of the excavation. Work is first begun on the 
invert at the point where the preceding ring of masonry ends, 
and is continued to the transverse strutting frame at the front 
of the excavation. As fast as the invert is completed, work is 
begun on the side walls. In very loose soils the longitudinal 
bars supporting the sides of the excavation are removed after 
the side walls are built; but in firmer soils they may be taken 
out one by one just ahead of the masonry, or in very firm soils 
it may be possible to remove them entirely before beginning 
the side walls. In all cases it is necessary to fill the space 
between the masonry and the walls of the excavation with rip- 
rap or earth. To build the roof arch the centers are first 
erected as described above, and the crown bars are removed as 
previously described by putting them ahead after the arch ring 
is completed. As with the side walls, the vacant space be- 
tween the arch ring and the roof of the excavation must 
be filled in. Usually earth or small stones are used for filling ; 
but in very loose soils it is sometimes the practice not to 
remove the poling-boards, but to support them by short brick 
pillars resting on the arch ring and then to fill around these 
pillars. 



THE FULL SECTION MPriHOD 161 

Hauling. — To haul away the material and take in supplies, 
tracks are laid on the invert masonry. Generally the perma- 
nent tracks are laid as fast as the lining is completed. A short 
section of temporary track is used to extend this permanent 
track close to the work of the advanced drift. 

Advantages and Disadvantages. — The great advantage of the 
English method of tunneling is that the masonry lining is 
built in one piece from the foundations to the crown, making- 
possible a strong, homogeneous construction. It also pos- 
sesses a decided advantage because of the simple methods of 
hauling which are possible : there being no differences of level 
to surmount, no hoisting of cars nor trans-shipments of loads 
are necessary. The cliief disadvantage of the method is that 
the excavators and masons work alternately, thus making the 
progress of the work slower perhaps than in any other method 
of tunneling commonly employed under similar conditions. 
This disadvantage is overcome to a considerable extent when 
the tunnel is excavated by shafts, and the work at the different 
headings is so arranged that the masons or excavators when 
freed from duty at one heading may be transferred to another 
where excavation or lining is to be done as the case may be. 
Another disadvantage of the EngUsh method arises from the 
excavation of the full section at once, which in unstable soils 
necessitates strong and careful strutting, and increases the 
danger of caving. The fact also that the arch ring has to 
carry the weight of the crown bars, and their loading at one 
end wliile the masonry is green, increases the chances of the 
arch being distorted. 

Conclusion. — The English method of tunneling in its entirety 
is confined in actual practice pretty closely to the country from 
which it receives its name. A possible extension of its use 
more generally is considered by many as likely to follow the 
development of a successful excavating machine for soft 
material. The space afforded by the opening of the full sec- 
tion at once, especially adapts the method to the use of exca- 



162 



TUNNELING 



vators like, for example, the endless chain bucket excavator 
used on the Central London Ry., and illustrated in Fig. 12. 
The method also furnishes an excellent opportunity for electric 
hauling and lighting during construction. 

The English method of tunneling has been used in building 
the Hoosac, Musconetcong, Allegheny, Baltimore and Potomac, 
and other tunnels in America. The names of the European 
tunnels built by this method are too numerous to mention here. 

AUSTRIAN METHOD. 

The Austrian full-section method of tunneling through soft 
ground was first used in constructing the Oberau tunnel on the 
Leipsic and Dresden R.R., in Austria in 1837. It consists in 
excavating the full section and building up the lining masonry 
from the foundations as in the English, but with the important 
exception that the invert is built last instead of first in all cases 
except where the presence of very loose soil requires its con- 
struction first. A still more important difference in the two 
methods is that the excavation is carried out in smaller sections 
and is continuous in the Austrian method instead of alternating 
with the mason work as it does in the English method. 



/< 


2 


■^ 


h 


3 


y] 


\ 7 
8 \ 


1 


7 / 
/ ^ 



f^ 


2 


\ 


3 


6 


I 


6 



Figs. 86 and87. — Diagrams Showing Sequence of Excavation in Austrian Method 

of Tunneling. 

Excavation. — The excavation in the Austrian method begins 
by driving the bottom center drift No. 1, Fig. 86, rising from 
the floor of the tunnel section nearly to the height of the 



THE FULL SPXTION METHOD 163 

springing lines of the roof arch, \yhen this drift has been 
driven ahead a distance varying from 12 ft. to 20 ft. or some- 
times more, the excavation of the center top heading No. 2 is 
driven for the same distance. The next operation is to remove 
})art No. 3, thus forming a central passage the full depth of the 
tunnel section at the center. This trench is enlarged by 
removing parts Nos. 4, 5, 6, 7, and 8 in the order named until 
the full section is opened. A modification of this plan of 
excavation is shown by Fig. 87 which is used in firm soils. 

Strutting. — Each part of the section is strutted as fast as 
it is excavated. The center bottom drift first excavated is 
strutted by laying a transverse sill across the floor, raising 
two side posts from it, and capping them with a transverse 
timber ^ having its ends projecting beyond the side posts and 
hal\ed as shown by Fig. 88. The top center heading No. 2, 
which is next excavated, is strutted by means of two side posts 
resting on blocking and carrying a transverse cap as also shown 
by Fig. 88. Sometimes the side posts in the heading strutting- 
frames are also carried on a transverse sill as are those of the 
bottom drift. This construction is usually adopted in loose 
soils. When the sill is employed, the middle part, No. 3, is 
strutted by inserting side posts between the bottom of the top 
sill and the cap of the frame in the drift below. When, how- 
ever, the posts of the top heading frame are carried on blocking, 
it is the practice to replace them with long posts rising from 
the cap of the bottom drift frame to the cap of the top heading 
frame. Further, when the intermediate sill is employed at the 
bottom level of the top heading it projects beyond the side 
posts and has its ends halved. 

After the completion of the center trench strutring the next 
task is to strut parts Nos. 4 and 5. This is done by continuing 
the upper sill hy means of a timber having one end halved to 
join with the projecting end of the sill in position. Tliis ex- 
tension timljer is shown at a, Fig. 89. The next operation is 
to place the timber h, having one end resting on the cap-piece 



164 



TUNNELING 



of the top heading frame and the other beveled and resting on 
the top of the sill a near the end. The timber h is laid tangent 
to the curve of the roof arch, and to support it against flexure 
the strut c is inserted as shown. To support the thrust of this 





strut the additional post d is 
inserted and the original bol^ 
torn heading frame is rein- 
forced as shown. The next 
step is to insert the strut e, 
and when this and the previ- 
ous construction are dupli- 
cated on the opposite side of 
the tunnel section we have 
the strutting of the parts Nos. 
1 to 5, inclusive, complete. 
Part No. 6 is then removed 
and strutted by extending the 
bottom drift cap-piece by a 
timber similar to timber a above, and then by inserting a side 
strut between the outer ends of these two timbers, as indicated 
by Fig. 90. As the final parts, Nos. 7 and 8, are removed, the 
inclined prop a, Fig. 90, is inserted as shown. When the soil 



Figs. 88 to 90. — Sketches Showing Construc- 
tion of Strutting, Austrian Method. 



THE FULL SECTION METHOD 



165 



is loose some of the members of the framework are doubled 
and additional bracing is introduced as shown by Fig. 90. 

The frames just described are placed at intervals of about 
4 ft. along the excavation, and are braced apart by horizontal 
struts. Some of the longitudinal bearing beams, as at ^, Fig. 
90, also extend through two or three frames, and help to tie 
them together. Finally, the longitudinal poling-boards extend- 
ing from one frame to the next along the walls of the excava- 
tion serve to connect them together. The short transverse 
beam c. Fig. 90, located just above the floor of the invert, 
serves to carry the planking upon which the train car tracks 
are laid. Besides the timber strutting peculiar to the Austrian 
method, the Rziha iron strutting described in a previous chapter 
is frequently used in tunneling by the Austrian process. 

Centers. — The two forms of centers used in the English 
method of tunneling are also 
used in the Austrian method. 
One of the methods of support- 
ing these centers is shown by 
Fig. 91. The tie-beam of the 
center rests on longitudinal tim- 
bers carried by the strutting 
frames and intermediate props. 
In single-track tunnels it is the 
frequent practice also to carry 
the ends of the tie-beams in re- 
cesses left in the side wall ma- 
sonry, with intermediate props 
inserted to prevent flexure at 
the center. When the Rziha 
iron strutting is employed, it also 
serves for the centering upon which the arch masonry is built. 

Masonry. — In the Austrian system of tunneling, the lining 
is built from the foundations of the side walls upward to the 
crown of the roof arch in lengths in consecutive rings equal to 




Fig. 91.— Sketch Showing Manner of 
Constructing the Lining Masonry, 
Austrian Method. 



166 TUNNELING 

the lengths of the consecutive openings of the full section, or 
from 12 ft. to 20 ft. long. Except in infrequent cases in very 
loose materials the invert is the last part of the masonry to be 
built, since to build it first requires the removal of the strutting 
which cannot easily or safely be accomplished until the side walls 
and roof arch are completed. As the side wall foundations are 
built, however, their interior faces are left inclined, as shown 
by Figs. 90 and 91, ready for the insertion of the invert, and 
are meanwhile kept from sliding inward by the insertion of 
blocking between them and the bottom of the strutting. Fig. 
91 shows the nature of this blocking, and also the manner in 
which the side wall and roof arch masonry is carried upward. 
Finally when the roof arch is keyed and the centers are struck, 
the strutting is taken down and the invert is built. 

Advantages and Disadvantages. — The principal advantages 
claimed for the Austrian method of tunneling are: (1) The 
excavation being conducted by driving a large number of con- 
secutive small galleries, which are immediately strutted, there 
is little disturbance of the surrounding material; (2) the 
polygonal type of strutting adopted is easily erected and of 
great strength against symmetrical pressures ; (3) the masonry, 
being built from the foundations up, is a single homogeneous 
structure, and is thus better able to withstand dangerous pres- 
sures ; (4) the excavation is so conducted that the masons 
and excavators do not interfere, and both can work at the same 
time. The disadvantages which the method possesses are : (1) 
The strutting, while very strong under symmetrical pressures, 
either vertical or lateral, is distorted easily by unsymmetrical 
vertical or lateral pressures, and by pressure in the direction of 
the axis of the tunnel; (2) the construction of the invert last 
exposes the side walls to the danger of being squeezed together, 
causing a rotation of the arch of the nature discussed in de- 
scribing the Belgian method of tunneling. 



SPECIAL TREACHEROUS GROUND METHOD 167 



CHAPTER XVI. 

SPECIAL TREACHEROUS GROUND METHOD; 
ITALIAN METHOD; QUICKSAND TUN- 
NELING; PILOT METHOD. 



ITALIAN METHOD. 

The Italian method of tunneling was first employed in con- 
structing the Cristina tunnel on the Foggia & Benevento R.R. 
in Italy. This tunnel penetrated a laminated clay of the most 
treacherous character, and after various other soft-ground 
methods of tunneling had been tried and had failed, Mr. Procke, 
the engineer, devised and used successfully the method which 
is now known as the Italian or Cristina method. The Italian 
method is essentially a treacherous soil method. It consists in 
excavating the bottom half of the section by means of several 
successive drifts, and building the invert and side walls ; the 
space is then refilled and the upper half of the section is exca- 
vated, and the remainder of the side walls and the roof arch 
are built ; finally, the earth filling in the lower half of the 
section is re-excavated and the tunnel completed. The method 
is an expensive one, but it has proved remarkably successful in 
treacherous soils such as those of the Apennine Mountains, 
in which some of the most notable Italian tunnels are located. 
It is, moreover, a single-track tunnel method, since any soil 
which is so treacherous as to warrant its use is too treacherous 
to permit an opening to be excavated of sufficient size for a 
double-track railway, except by the use of shields. 

Excavation. — The plan of excavation in the Italian method 
is shown by the diagram Fig. 92. Work is begun by driving 



168 



TUNNELING 




Pig. 92. — Diagram Show- 
ing Sequence of Excava- 
tion in Italian Metliod of 
Tunneling. 



the center bottom heading No. 1, and this is widened by taking 
out parts No. 2. Finally part No. 3 is removed, and the lower 
half of the section is open. As soon as the invert and side 
wall masonry has been built in this excavation, parts No. 2 
are filled in again with earth. The exca- 
vation of the center top heading No. 4 is 
then begun, and is enlarged by removing 
the earth of part No. 5. The faces of this 
last part are inclined so as to reduce their 
tendency to shde, and to permit of a 
greater number of radial struts to be 
placed. Next, parts No. 6 are excavated, 
and when this is done the entire section, 
except for the thin strip No. 7, has been 
opened. At the ends of part No. 7 nar- 
row trenches are sunk to reach the tops of the side walls 
already constructed in the lower half of the section. The 
masonry is then completed for the upper half of the section, 
and part No. 7 and the filling in parts No. 2 are removed. 
The various drifts and headings and ^.-----^ 

the parts excavated to enlarge them ^/" 

are seldom excavated more than from 
6 ft. to 10 ft. ahead of the lining. 

Strutting. — The bottom center 
drift, which is first driven, is strutted 
by means of frames consisting of side 
posts resting on floor blocks and car- 
rying a cap-piece. Poling-boards are 
placed around the walls, stretching 
from one frame to the next. As 
soon as the invert is sufficiently completed to permit it, the 
side posts of the strutting frames are replaced by short struts 
resting on the invert masonry as shown by Fig. 93. To permit 
the old side posts to be removed and the new shorter ones to 
be inserted, the cai>piece of the frame is temporarily supported 




Fia. 93. — Sketch Showing Strut- 
ting for Lower Part of Section. 



SPECIAL TREACHEROUS GROUND METHOD 



169 



by inclined props arranged as shown by Fig. 97. When parts 
No. 2 are excavated the roof is strutted by inserting the trans- 
verse caps ^, Fig. 93, the outer ends of which are carried by the 
system of struts 6, c, d^ and e. The longitudinal poling-boards 
supporting the ceiling and walls are held in place by the cap 
a and the side timber e. To stiffen the frames longitudinally 
of the tunnel, horizontal longitudinal struts are inserted between 
them. 

The excavation of the upper half of the tunnel section is 
strutted as in the Belgian method, with radial struts carrying 
longitudinal roof bars and transverse poling-boards. On ac- 
count of the enormous pressures developed by the treacherous 
soils in w^hich only is the Italian method employed, the radial 
strutting frames and crown bars must be of great strength, 





Figs. 94 and 95. — Sketches Showing Construction of Centers, Italian Method. 

while the successive frames must be placed at frequent intervals, 
usually not more than 3 ft. After the masonry side walls have 
been built in the lower part of the excavation, longitudinal 
planks are laid against the side posts of the center bottom 
drift frames, to form an enclosure for the filling-in of parts 
No. 2. The object of this filling is principally to prevent 
the squeezing-in of the side walls. 

Centers. — Owing to the great pressures to Ije resisted in the 
treacherous soils in which the Italian method is used, the con- 
struction of the centers has to be very strong and rigid. Figs. 
94 and 95 show two common types of center construction used 
with this method. The construction shown in Fie. 94 is a 
strong one where only pressures normal to the axis of the 
tunnel have to be withstood, but it is likely to twist under 



170 TUNNELING 

pressures parallel to the axis of the tunnel. In the construc- 
tion shown by Fig. 95, special provision is made to resist 
pressures normal to the plane of the center or twisting pres- 
sures, by the strength of the transverse bracing extending hori- 
zontally across the center. 

Masonry. — The construction of the masonry lining begins 
with the invert, as indicated by Fig. 93, and is carried up to the 
roof of parts No. 2, as already indicated, and is then discon- 
tinued until the upper parts Nos. 4, 5, and 6 are excavated. 
The next step is to sink side trenches at the ends of part No. 7, 
which reach to the top of the completed side walls. This 
operation leaves the way clear to finish the side walls and to 
construct the roof arch in the ordinary manner of such work in 

tunneling. Since this method of 
tunneling is used only in very soft 
ground which yields under load, the 
usual practice is to construct the in- 
vert and side walls on a continuous 

Fig. 96. -Sketch Showing Invert foundation COUrSC of COUCrctC aS iu- 
and Foundation Masonry, Italian dicatcd bv Fisf. 96. The Hningf is 
Method. n 1 -T . . . 1 

usually built m successive rings, and 
the usual precautions are taken with respect to filling in the 
voids behind the lining. The thickness of the lining is based 
upon the figures for laminated clay of the third variety given 
in Table II. 

Hauling The system of hauling adopted with this method 

of tunneling is very simple, since the excavation of the various 
parts is driven only from 6 ft. to 10 ft. ahead, and the work pro- 
gresses slowly to allow for the construction of the heavy strutting 
required. To take away the material from the center bottom 
drift, narrow-gauge tracks carried by cross-beams between the 
side posts above the floor line are employed. This same 
narrow-gauge line is employed to take away a portion of parts 
No. 2, the remaining portion being left and used for the refill- 
ing after the bottom portion of the lining has been built, as 




SPECIAL TREACHEROUS GROUND METHOD 



171 



previously described. The upper half of the section being ex- 
cavated, as in the Belgian method, the system of hauling with 
inclined planes to the tunnel floor below, which is a character- 
istic of that method, may be employed. It is the more usual 




Fig. 97.— Sketch Showing Longitudinal Section of a Tunnel under Construction, 
Italian Method. 

practice, however, since the excavation is carried so little a dis- 
tance ahead and progresses so slowly, to handle the spoil from 
the upper part of the section by wheelbarrows which dump it 
into the cars running on the tunnel floor below. Hand labor 
is also used to raise the construction 
materials used in building the upper sec- 
tion. The tracks on the tunnel floor, 
besides extending to the front of the ad- 
vanced bottom center drift, have right and 
left switches to be employed in removing 
the refilling in parts No. 2, the spoil from 
the upper part of the section, and the 
material of part No. 7. Fig. 97 is a longi- 
tudinal section showing the plan of exca- 
vation and strutting adopted with the Italian method. 

Modifications. — It often happens that the filling placed be- 
tween the side walls and the planking, which is practically the 
space comprised by parts No. 2, is not sufficient to resist the 
inward pressure of the walls, and they tip inward. In these 
cases a common expedient is to substitute for the earth filling 



/ ' 




5 


/•\ 


^ i 


' 




' 


' 


1 1 




2 


^ 



Fig. 98. — Sketch Showing 
Sequence of Excavation, 
Stazza Tunnel. 



172 



TUNXELINCJ 



a temporary masonry arch sprung between the side walls 
with its feet near the bottom of the walls, and its crown, 
just below the level of their tops, as shown by Fig. 101. 
This construction was employed in the 
Stazza tunnel in Italy. In this tunnel 
the excavation was begun by driving the 
center drift, No. 1, Fig. 98, and immedi- 
ately strutting it as shown by Fig. 99. 
The other parts, Nos. 2 and 3, completing 
the lower portion of the section, were then 
taken out and strutted. AVhile part No. 2 
Fig. 99. — Sketch Showing was being cxcavatcd at the bottom, and 
DriftstazfaT^^nfi^'''' the Center part of the invert built, the 
longitudinal crown bars carrying the roof 
of the excavation were carried temporarily by the inclined 
props shown by Fig. 100. After completing the invert and 
the side walls to a height of 2 or 3 ft., a thick masonry arch 
was sprung between the side walls, as shown in transverse 
section by Fig. 101, and in longitudinal section by Fig. 100. 
This arch braced the side walls against tipping inward, and 





Figs. 100 and 101.— Sketches Showing Temporary Strutting Arch Construction, 
Stazza Tunnel. 



carried short struts to support the crown bars. The haunches 
of the arch were also filled in with rammed earth. The upper 
half of the section was excavated, strutted, and lined as in 
the standard Italian method previously described. When the 
lining was completed, the arch inserted between the side walls 
was broken down and removed. 



SPECIAL TREACHEROUS GROUND METHOD 173 

Advantages and Disadvantages, — The great advantage claimed 
for the Italian method of tunneling is that it is built in two 
separate parts, each of which is separately excavated, strutted, 
and lined, and thus can be employed successfully in very 
treacherous soils. Its chief disadvantage is its excessive cost, 
which limits its use to tunnels through treacherous soils where 
other methods of timbering^ cannot be used. 



QUICKSAND TUNNELING. 

When an undergromid stream of water passes with force 
through a bed of sand it produces the phenomenon known as 
quicksand. This phenomenon is due to the fineness of the 
particles of sand and to the force of the water, and its activity 
is directly proportional to them. When sand is confined it 
furnishes a good foundation bed, since it is practically incom- 
pressible. To work successfully in quicksand, therefore, it is 
necessary to drain it and to confine the particles of sand so 
that they cannot flow away with the water. This observation 
suggests the mode of procedure adopted in excavating tunnels 
through quicksand, which is to drain the tunnel section by 
opening a gallery at its bottom to collect and carry away the 
water, and to prevent the movement or flowing of the sand by 
strutting the sides of the excavation with a tight planking. 

The sand having to be drained and confined as described, the 
ordinary methods of soft^ground tunneling must be employed, 
with the follo\ving modifications : 

(1) The first work to be performed is to open a bottom 
gallery to drain the tunnel. This gallery should be lined with 
boards laid close and braced sufficiently by interior frames to 
prevent distortion of the lining. The interstices or seams be- 
tween the lining boards should be packed with straw so as to 
permit the percolation of water and yet prevent tlie movement 
of the sand. 

(2) As fast as tlie excavation progresses its walls should 



174 TUNNELING 

be strutted by planks laid close, and held in position by interior 
framework; the seams between the plank should be packed 
with straw. 

(3) The masonry lining should be built in successive rings, 
and the work so arranged that the water seeping in at the sides 
and roof is collected and removed from the tunnel immediately. 

Excavation. — The best and most commonly employed method 
of driving tunnels through quicksand is a modification of the 
Belgian method. At first sight it may appear a hazardous work 
to support the roof arch, as is the characteristic of this method, 
on the unexcavated soil below, when this soil is quicksand, but 
if the sand is well confined and drained the risk is really not 
very great. Next to the Belgian method the German method 
is perhaps the best for tunneling quicksand. In these compari- 
sons the shield system of tunneling is for the time being left 
out of consideration. This method will be described in suc- 
ceeding chapters. Whenever any of the systems of tunneling 
previously described are employed, the first task is always to 
open a drainage gallery at the bottom of the section. 

Assuming the Belgian method is to be the one adopted, the 
first work is to drive a center bottom drift, the floor of which 
is at the level of the extrados of the invert. This drift is im- 
mediately strutted by successive transverse frames made up of 
a sill, side posts, and a cap which support a close plank strut- 
ting or lining, with its joints packed with straw. Between the 
side posts of each cross-frame, at about the height of the 
intrados of the invert, a cross-beam is placed ; and on these cross- 
beams a plank flooring is laid, w^hich divides the drift horizon- 
tally into two sections, as shown by Fig. 102; the lower section 
forming a covered drain for the seepage water, and the upper 
providing a passageway for workmen and cars. The bottom 
drift is driven as far ahead as practicable, in order to drain the 
sand for as great a distance in advance of the work as possible. 
After the construction of the bottom drainage drift the excava- 
tion proper is begun, as it ordinarily is in the Belgian method 



SPECIAL TREACHEROUS GROUND METHOD 



175 



y 




Fig. 102. — Sketch Showing 
Preliminary Drainage Gal- 
leries, Quicksand Method. 



by driving a top center heading, as shown by Fig. 102. This 
heading is deepened and widened after the manner usual to the 
Belgian method, until the top of the sec- 
tion is open down to the springing lines 
of the roof arch. To collect the seepage 
water from the center top heading it is 
provided with a center bottom drain con- 
structed like the drain in the bottom 
drift, as shown by Fig. 102. When the 
top heading is deepened to the level of 
the springing lines of the roof arch, its 
bottom drain is reconstructed at the new 
level, and serves to drain the full top 
section opened for the construction of the 
roof arch. This top drain is usually con- 
structed to empty into the drain in the bottom drift. 

Strutting. — The method of strutting the bottom drift has 
already been described. For the remainder of the excavation 
the regular Belgian method of radial roof strutting-frames is 

employed, as shown by Fig. 103. 
Contrary to what might be expected, 
the number of radial struts required 
is not usually greater than would be 
used in many other soils besides 
quicksand. Single-track railway tun- 
nels have been constructed through 
quicksand in several instances where 
the number of radial props required 
on each side of the center did not 
exceed four or five. It is necessary, 
however, to place the poling-boards 
very close together, and to pack the joints between them to 
prevent the inflow of the fine sand. In strutting the lower 
part of the section it is also necessary to support the sides with 
tight planking. This is usually held in place by longitudinal 




Fig. 103.— Sketch Showing Con- 
struction of Roof Strutting 
Quicksand Method. 



176 



TUNNELING 




Fig. 104,— Sketch Showing Construc- 
tion of Masonry Lining, Quicksand 
Method. 



bars braced by short struts against the inclined props employed 
to carry the roof arch when the material on which they origi- 
nally rested is removed. This side 
strutting is shown at the right 
hand of Fig. 104. 

Masonry. — As soon as the upper 
part of the section has been opened 
the roof arch is built with its feet 
resting on planks laid on the unex- 
cavated material below. This arch 
is built exactly as in the regular 
Belgian method previously de- 
scribed, using the same forms of 
centers and the same methods 
throughout, except that the poling- 
boards of the strutting are usually left remaining above the 
arch masonry. To prevent the possibility of water percolating 
through the arch masonry, many engineers also advise the 
plastering of the extrados of the arch with a layer of cement 
mortar. This plastering is designed to lead the water along 
the haunches of the arch and down behind the side walls. In 
constructing the masonry below the roof arch the invert is 
built first, contrary to the regular Belgian method, and the 
side walls are carried up on each side from the invert ma- 
sonry. Seepage holes are left in the invert masonry, and also 
in the side walls just above the intrados of the invert. At the 
center of the invert a culvert or drain is constructed, as shown 
by Fig. 104, inside the invert masonry. This culvert is com- 
monly made with an elliptical section with its major axis hori- 
zontal, and having openings at frequent intervals at its top. 
The thickness of the lining masonry required in quicksand is 
shown by Table II. 

Removing the Seepage Water. — After the tunnel is completed 
the water which seeps in through the weep-holes left in the ma- 
sonry passes out of the tunnel, following the direction of the 



SPECIAL TREACHEROUS GROUND METHOD 171 

descending grades. During construction, however, special 
means will have to be provided for removing the water from 
the excavation, their character depending upon the method of 
excavation and upon the grades of the tunnel bottom. When 
the excavation is carried on from the entrances only, unless the 
tunnel has a descendmg grade from the center toward each end, 
the tunnel floor in one heading will be below the level of the en- 
trance, or, in other words, the descending grade will be toward 
the point where Avork is going on, while at the opposite entrance 
the grade will be descending from the work. In the latter 
case the removal of the seepage water is easily accomplished by 
means of a drainage channel along the bottom of the excavation. 
In the former case the water which drains toward the front is 
collected in a sump, and if there is not too great a difference in 
level between this sump and the entrance, a siphon may be used 
to remove it. Where the siphon cannot be used, pumps are 
installed to remove the water. When the tunnel is excavated 
by shafts the condition of one high and one low front, as com- 
pared with the level at the shaft, is had at each shaft. Gene- 
rally, therefore, a sump is constructed at the bottom of the 
shaft ; the culvert from the high front drains directly to the 
shaft sump, while the water from the low-front sump is either 
siphoned or pumped to the shaft sump. From the shaft sump 
the water is" forced up the shaft to the surface by pumps. 

THE PILOT METHOD. 

The pilot system of tunneling has been successfully em- 
ployed in constructing soft-ground sewer tunnels in America 
by the firm of Anderson & Barr, which controls the patents. 
The most important work on which the system has been em- 
ployed is the main relief sewer tunnel built in Brooklyn, N.Y., 
in 1892. This work comprised 800 ft. of circular tunnel 15 ft. 
in diameter, 4400 ft. 14 ft. in diameter, 3200 ft. 12 ft. in 
diameter, and 1000 ft. 10 ft. in diameter, or 9400 ft. of tunnel 



178 



TUNNELING 



altogether. The method of construction by the pilot system is 
as follows : 

Shafts large enough for the proper conveyance of materials 
from and into the tunnel are sunk at such places on the line of 
work as are most convenient for the purpose. From these 
shafts a small tunnel, technically a pilot, about 6 ft. in diameter, 
composed of rolled boiler iron riveted to light angle irons on 
four sides, perforated for bolts, and bent to the required radius 
of the pilot, is built into the central part of the excavation on 
the axis of the tunnel. This pilot is generally kept about 30 ft. 
in advance of the completed excavation, as shown by Fig. 105. 
The material around the exterior of the pilot is then excavated, 
using the pilot as a support for braces which radiate from it and 




Bracing." ^ Arch' Constrijction. 

Fig. 105. — Sketch Showing Pilot Method of Tunneling. 

secure in position the plates of the outside shell which holds 
the sand, gravel, or other material in place until the concentric 
rings of brick masonry are built. Ribs of T-iron bent to the 
radius of the interior of the brick work, and supported by the 
braces radiating from the pilot, are used as centering supports 
for the masonry. On these ribs narrow lagging-boards are laid 
as the construction of the arch proceeds, the braces holding the 
shell plates and the superincumbent mass being removed as the 
masonry progresses. The key bricks of the arches are placed 
in position on ingeniously contrived key-boards, about 12 ins. in 
width, which are fitted into rabbeted lagging-boards one after 
another as the key bricks are laid in place. After the masonry 
has been in place at least twenty-four hours, allowing the cement 



SPECIAL TREACHEROUS GROUND METHOD 1 < 9 

mortar time to set, the braces, ribs, and lagging which support 
it are removed. In the meantime the excavation, bracing, pilot, 
and exterior shell have been carried forward, preparing the way 
for more masonry. The top plates of the shell are first placed 
in position, the material being excavated in advance and sup- 
ported by light poling-boards ; then the side-plates are butted 
to the top and the adjoining side-plates. In the pilot the plates 
are united continuously around the perimeter of the circle, 
while in the exterior shell the plates are used for about one- 
third of the perimeter on top, unless treacherous material is 
encountered, when the plates are continued down to the spring- 
ing lines of the arch. This iron lining is left in place. The 
bottom is excavated so as to conform to the exterior lines of 
the masonry. The excavation follows so closely to the outer 
lines of the normal section of the tunnel that very little loss 
occurs, even in bad material; and there is no loss where suffi- 
cient bond exists in the material to hold it in place until the 
poling-boards are in position. 

In the Brooklyn sewer tunnel work, previously mentioned, 
the pilot was built of steel plates | in. thick, 12 ins. wide, and 
37^ ins. long, rolled to a radius of 3 ft. Steel angles 4 x 4|^ ins. 
were riveted along all four sides of each plate, and the plates 
were bolted together by |-in. machine-bolts. The plates weighed 
136 lbs. each, and six of them were required to make one com- 
plete ring 6 ft. in diameter. In bolting them together, iron 
shims were placed between the horizontal joints to form a 
footing for the wooden braces for the shell, which radiate from 
the pilot. The shell plates of the 15-ft. section of the tunnel 
were of No. 10 steel 12 ins. wide and 37 ins. long, with steel 
angles 2^ x 2^ x f ins., riveted around the edges the same as for 
the pilot, and put together with f-in. bolts. These plates 
weighed 61 lbs. each, and eighteen of them were required to 
make one complete ring 15 ft. in diameter. The plates for the 
12-ft. section were No. 12 steel 12 ins. wide with 2x 2 x i-in. 
angles. Seventeen plates were required to make a complete ring. 



180 TUNNELING 



CHAPTER XVII. 

OPEN-CUT TUNNELING METHODS; TUNNELS 

UNDER CITY STREETS ; BOSTON SUBWAY 

AND NEW YORK RAPID TRANSIT. 



OPEN-CUT TUNNELING. 

When a tunnel or rapid-transit subway has to be constructed 
at a small depth below the surface, the excavation is generally 
performed more economically by making an open cut than by 
subterranean tunneling proper. The necessary condition of 
small depth which makes open-cut tunneling desirable is most 
generally found in constructing rapid-transit subways or tun- 
nels under city streets. This fact introduces the chief difficul- 
ties encountered in such work, since the surface traffic makes it 
necessary to obstruct the streets as little as possible, and has 
led to the development of the several special methods commonly 
employed in performing it. These methods may be classed as 
follows : (1) The longitudinal trench method, ulsing either a 
single wide trench or two narrow parallel trenches; (2) the 
transverse trench method. 

Single Longitudinal Trench. — The simplest manner by which 
to construct open-cut tunnels is to open a single cut or trench 
the full width of the tunnel masonry. This trench is strutted 
by means of side sheetings of vertical planks, held in place by 
transverse braces extending across the trench and abutting 
against longitudinal timbers laid against the sheeting plank. 
The lining is built in this trench, and is then filled around and 
above with well-rammed earth, after which the surface of the 
ground is restored. An especial merit of the single longitudi- 
nal trench method of open-cut tunneling is that it permits the 



OPEN-CUT TUNNELING METHODS 



181 




Fig. 106. — Diagram Showing Se- 
quence of Construction in Open- 
Cut Tunnels. 



construction of the lining in a single piece from the bottom up, 

thus enabling better workmanship and stronger construction 

than when the separate parts are built at different times. The 

great objection to the method when 

it is used for building subways 

under city streets is, that it occupies 

so much room that the street usually 

has to be closed to regular traffic. 

For this reason the sino^le long-i- 

tudinal trench method is seldom 

employed, except in those portions 

of city subways which pass under 

public squares or parks where room 

is plenty. 

Parallel Longitudinal Trenches. — 
The parallel longitudinal trench method of open-cut tunneling 
consists in excavating two narrow parallel trenches for the side 
walls, leaving the center core to be removed after the side 
walls have been built. The diagram, Fig. 106, shows the 

sequence of opera- 
tions in this method. 
The two trenches No. 
1 are first excavated 
a little wider than the 
side wall masonry, 
and strutted as shown 
by Fig. 107. At the 
bottoms of these 
trenches a foundation 
course of concrete is 
laid, as shown by Fig. 
108, if the ground is 
soft; or the masonry is started directly on the natural material, 
if it is rock. P>om the foundations the walls are carried up to 
the level of the springing lines of the roof arch, if an arch is 




Pig. 107.— Sketch Showing Method of Timhering Open- 
Gut Tunnels, I)oul)le Parallel Trench Method. 



182 TUNNELING 

used ; or to the level of its ceiling, if a flat roof is used. After 
the completion of the side walls, the portion of the excavation 
shown at No. 2, Fig. 106, is removed a sufficient depth to en- 
able the roof arch to be built. When the arch is completed, it 
is filled above with well-rammed earth, and the surface is re- 
stored. The excavation of part No. 3 inclosed by the side 
walls and roof arch is carried on from the entrances and from 
shafts left at intervals along the line. 

A modification of the method just described was employed 
in constructing the Paris underground railways. It consists in 
excavating a single longitudinal trench along one side of the 
street, and building the side wall in it as previously described. 
When this side wall is completed to the 
roof, the right half of part No. 2, Fig. 106, 
is excavated to the line AB, and the right- 
hand half of the roof arch is built. The 
space above the arch is then refilled and the 
surface of the street restored, after which 
the left-hand trench is dug and the side 
_ „ wall and roof-arch masonry is built lust as 

Fig. 108. — Side -Wall ^ J J 

Foundation Con- in the opposltc half. Generally the work 

struction Open-Cut . j_ i i • i j.i x 

Tunnels. ^^ prosccutcd by opening up lengths oi 

trench at considerable intervals along the 
street and alternately on the left- and right-hand sides. By 
this method one-half of the street width is everywhere open 
to traffic, the travel simply passing from one side of the street 
to the other to avoid the excavation. When the lining has 
been completed, the center core of earth inclosed by it is 
removed from the entrances and shafts, leaving the tunnel 
finished except for the invert and track construction, etc. 

Transverse Trenches. — The transverse trench or " slice " 
method of open-cut tunneling has been employed in one work, 
the Boston Subway. This method is described in the specifica- 
tions for the work prepared by the chief engineer, Mr. H. A. 
Carson, M. Am. Soc. C. E., as follows : — 




OPEN-CUT TUNNELING METHODS 183 

"Trenches about 12 ft. wide shall be excavated across the 
street to as great a distance and depth as is necessary for the 
construction of the subway. The top of this excavation shall 
be bridged during the night by strong beams and timbering, 
whose upper surface is flush with the surface of the street. 
These beams shall be used to support the railway tracks as well 
as the ordinary traffic. In each trench a small portion or slice 
of the subway shall be constructed. Each slice of the subway 
thus built is to be properly joined in due time to the contiguous 
slices. The contractor shall at all times have as many slice- 
trenches in process of excavation, in process of being filled with 
masonry, and in process of being back-filled with earth above 
the completed masonry, as is necessary for the even and steady 
progress of the work towards completion at the time named in 
the contract." 

In regard to the success of this method Mr. Carson, in his 
fourth annual report on the Boston Subway work, says : 

*' The method was such that the street railway tracks were 
not disturbed at all, and the whole surface of the street, if de- 
sired, was left in daytime wholly free for the normal traffic." 

Tunnels on the Surface. — It occasionally happens when 
filling-in is to take place in the future, or where landslides 
are liable to bury the tracks, that a railway tunnel has to be 
built on the surface of the ground. In such cases the construc- 
tion of the tunnel consists simply in building the lining of the 
section on the ground surface with just enough excavation to 
secure the proper grade and foundation. Generally the lining 
is finished on the outside with a waterproof coating, and is 
sometimes banked and partly covered with earth to protect the 
masonry from falling stones and similar shocks from other 
causes. A recent example of tunnel construction of this char- 
acter was described in " Engineering News " of Sept. 8, 1898. 
In constructing the Golden Circle Railroad, in the Cripple Creek 
mining district of Colorado, the line had to be carried across a 
valley used as a dumping-ground for the refuse of the surround- 



184 TUNNELING 

ing mines. To protect the line from this refuse, the engineer 
constructed a tunnel lining consisting of successive steel ribs, 
filled between with masonry. 

Concluding Remarks. — From the fact that the open-cut 
method of tunneling consists first in excavating a cut, and sec- 
ond in covering this cut to form an underground passageway, 
it has been named the " cut-and-cover " method of tunneling. 
The cut-and-cover method of tunneling is almost never employed 
elsewhere than in cities, or where the surface of the ground has 
to be restored for the accommodation of traffic and business. 
When it is not necessary to restore the original surface, as is 
usually the case with tunnels built in the ordinary course of 
railway work, it would obviously be absurd to do so except in 
extraordinary cases. In a general way, therefore, it may be said 
that the cut-and-cover method of construction is confined to the 
building of tunnels under city streets ; and the discussion of 
this kind of tunnels follows logically the general description of 
the open-cut method of tunneling which has been given. 

TUNNELS UNDER CITY STREETS. 

The three most common purposes of tunnels under city 
streets are : to provide for the removal of railway tracks from 
the street surface, and separate the street railway traffic from 
the vehicular and pedestrian traffic; to provide for rapid 
transit railways from the business section to the outlying 
residence districts of the city ; and to provide conduits for sew- 
age or subways for water and gas mains, sewers, wires, etc. 
Within recent years the greatest works of tunneling under city 
streets have been designed and carried out to furnish improved 
transit facilities. 

Conditions of Work. — The construction of tunnels under city 
streets may be divided into two classes, which may be briefly 
defined as shallow tunnels and deep tunnels. Shallow tunnels, 
or those constructed at a small depth beneath the surface, are 



OPEN-CUT TUNNELING METHODS 185 

usually built by one of the cut-and-cover methods ; deep 
tunnels, or those built at a great depth, beneath the surface 
are constructed by any of the various methods of tunneling 
described in this book, the choice of the method depending 
upon the character of the material penetrated, and the local 
conditions. 

In building tunnels under city streets the first duty of the 
engineer is to disturb as little as possible the various existing 
structures, and the activities for which these structures and the 
street are designed. The character of the difficulties encoun- 
tered in performing this duty will depend upon the depth at 
which the tunnel is driven. In constructing shallow tunnels 
by the cut-and-cover method care has to be taken first of all 
not to disturb the street traffic any more than is absolutely 
necessary. This condition precludes the single trench method 
of open cut tunneling in all places where the street traffic is at 
all dense, and compels the engineer to use the parallel trench 
method employed in Paris, as previously described, or else the 
transverse trench or slice method employed in the Boston 
Subway. 

Both of these methods have to be modified when the work 
is done on streets having underground trolley and cable roads, 
and in which are located gas and water pipes, conduits for 
wires, etc. Where underground trolley or cable railways are 
encountered, a common mode of procedure is to excavate 
parallel side trenches for the side walls, and turn the roof arch 
until it reaches the conduit carrying the cables or wires. The 
earth is then removed from beneath the conduit structure in 
small sections, and the arch completed as each section is 
opened. As fast as the arch is completed the conduit struc- 
ture is supported on it. Where pipes are encountered they 
may be supported by means of chains, suspending them from 
heavy cross-beams, or by means of strutting, or they may be 
removed and re])uilt at a new level. Generally the conditions 
require a different solution of this problem at different points. 



186 TUNNELING 

Another serious difficulty of tunneling under city streets 
arises from the danger of disturbing the foundations of the 
adjacent buildings. This danger exists only where the depth 
of the tunnel excavation extends below the depth of the build- 
ing foundations, and where the material penetrated is soft 
ground. Where the tunnel penetrates rock there is no danger 
of disturbing the building foundations. To prevent trouble of 
this character requires simply that the excavation of the 
tunnel be so conducted that there is no inflow of the surround- 
ing material, which may, by causing a settlement of the neigh- 
boring material, allow the foundations resting on it to sink. 

The Baltimore Belt tunnel, described in a preceding chap- 
ter, is an example of the method of work adopted in construct- 
ing a tunnel under city streets through very soft ground. 
This may be classed as a deep tunnel. Another method of 
deep tunneling under city streets is the shield method, ex- 
amples of which are given in a succeeding chapter. Two 
notable examples of cut-and-cover methods of tunneling are 
the Boston Subway and the New York Rapid Transit Ry., a 
description of which follows. 

Boston Subway. — The Boston Subway may be defined as the 
underground terminal system of the surface street railway 
system of the city, and as such it comprises various branches, 
loops, and stations. The subway begins at the Public Garden 
on Boylston St., near Charles St., and passes with double 
tracks under Boylston St. to its intersection with Tremont St., 
where it meets the other double-track branch, passing under 
Tremont St. and beginning at its intersection with Shawmut 
Ave. From their intersection at Tremont and Boylston streets 
the two double-track branches proceed under Tremont St. with 
four tracks to Scollay Square. At Scollay Square the subway 
divides again into two double-track branches, one passing 
under Hanover St., and the other under Washington St. At 
the intersection of Hanover and Washington streets the twO' 
double-track branches combine again into a four-track line„ 



OPEN-CUT TUNJ^ELING METHODS 187 

which runs under Washington St. to its terminus at Hay- 
market Square, where it comes to the surface by means of an 
incline. The subway, therefore, has three portals or entrances, 
located respectively at Boylston St., Shawmut Ave., and Hay- 
market Square. It also has five stations and two loops, the 
former being located at Boylston St., Park St., Scollay Square, 
Adams Square, and Haymarket Square, and the latter at Park 
St. and Adams Square. The total length of the subway is 
10,810 ft. 

Material Penetrated. — The material met with in construct- 
ing the subway is alluvial in character, the lower strata being 
generally composed of blue clay and sand, and the upper strata 
of more loose soil, such as loam, oyster shells, gravel, and peat. 
At many points the material was so stable that the walls of 
the excavation would stand vertical for some time after excava- 
tion. Surface water was encountered, but generally in small 
quantities, except near the Boylston St. portal, where it was 
so plentiful as to cause some trouble. 

Cross- Section. — The subway being built for two tracks in 
some places and for four tracks in other places, it was neces- 
sary to vary the form and dimensions of the cross-section. 
The cross-sections actually 
adopted are of three types. 
Fig. 109 shows the section 
known as the wide arch type, 
in which the lining is solid 
masonry. The second type 
was known as the double- 
barrel section, and is shown 
by Fig. 110. Tlie third type 
of section is shown by Fig. 111. The lining consists of steel 
columns carrying transverse roof girders ; the roof girders 
being filled between with arches, and the wall columns having 
concrete walls between them. Tlie wide-arch type and the 
double-barrel type of sections were emph)yed in some portions 




Fio. 109.— Wide Arch Section, Boston Subway. 



188 



TUNNELING 



of the Tremont St. line, where the traffic was very dense, 
since it was possible to construct them without opening the 
street. Much of the wide arch line was constructed by the 
use of the roof shield, which is described in the succeeding 
chapter on the shield system of tunneling. 

Methods of Construction. — Several different methods were 
employed in constructing the subway. Where ample space 
was available, the single wide trench method of cut-and-cover 




Fig. 110.— Double Barrel Section, Boston Subway. 



construction was employed, the earth being removed as fast as 
excavated. In the streets, except where regular tunneling was 
resorted to, the parallel trench or transverse trench cut-and- 
cover methods were employed. 

In the transverse trench method, trenches about 12 ft. wide 
were excavated across the street, their length being equal to 
the extreme transverse width of the tunnel lining, and their 
depth being equal to the depth of the tunnel floor. These 
trenches were begun during the night, and immediately roofed 



OPEN-CUT TUNNELESTG METHODS 



189 



over with a timber platform flush with the street surface. 
Under these platforms the excavation was completed and the 
lining built. As each trench or " slice " was completed, the 
street above it was restored and the platform reconstructed 



■ConcreTt 



. Tar Conc^rs--X2^ 



..:,',.",'..' S-Waterproofi'nqtPtaster 'Fine BroHen stone-- . 

Cross Section of Side Wall. haymarket square Cross Section of Roof. 



Drain 




Fig. 111. — Four Track Rectangular Section, Boston Subway. 

over the succeeding trench or slice. During the construction 
of each slice the street traffic, including the street cars, was 
carried by the timber platform. 

In the parallel trench method, short parallel trenches were 
dug for the opposite side walls, and also for the intermediate 




-^ "~" Waterproofing-' " 

Fig. 112. — Section Showing Slice Method of Construction, Boston Subway. 

columns, and completely roofed over during the night. Under 
this roofing the masonry of the side walls and column founda- 
tions and the columns themselves were erected. When the 
side walls and columns had been erected, the surface of the 
street between them was removed, the roof beams laid, and a 



190 TUNNELING 

platform covering erected, as shown by Fig. 112. This roofing 
work was also done at night. The subsequent work of build- 
ing the roof arches, removing the remainder of the earth, and 
constructing the invert, Avas carried on underneath the plat- 
form covering which carried the street traffic in the meantime. 
The successive repetition of the processes described con- 
structed the subway. 

Where the traffic was very dense on the street above, tunnel- 
ing was resorted to. For small portions of this work the ex- 
cavation was done in the ordinary way, using timber strutting, 
but much the greater portion of the tunnel work was performed 
by means of a roof shield. In the latter case, the side w^alls 
were first built in small bottom side drifts and were fitted with 
tracks on top to carry the roof shield. The construction and 
operation of this shield are described fully in the succeeding 
chapter on the shield system of tunneling. 

Masonry. — The masonry of the inclined approaches to the 
subway consists simply of two parallel stone masonry retaining 
walls. In the wide-arch and double-barrel tunnel sections, the 
side walls are of concrete and the roof arches are of brick masonry. 
In the other parts of the subway the masonry consists of brick 
jack arches sprung between the roof beams and covered with 
concrete, of concrete walls embedding the side columns, and 
of the concrete invert and foundations for the columns. Figs. 
109 to 112 inclusive show the general details of the masonry 
work for each of the three sections. The inside of the lining 
masonry is painted throughout with white paint. 

Stations. — The design and construction of the stations for 
the Boston Subway were made the subjects of considerable 
thought. All the stations consist of two island platforms of 
artificial stone having stairways leading to the street above. 
The platforms are made 1 ft. higher than the rails. The station 
structure itself is built of steel columns and roof beams with 
brick roof arches and concrete side walls. Its interior is lined 
with white enameled tiles. The intermediate columns are cased 



OPEN-CUT TUNNELING METHODS 191 

with wood, and have circular wooden seats at their bottoms. 
Each stairway is covered by a light housing, consisting of a 
steel framework witii a copper covering and an interior wood 
and tile linish. 

Ventilation. — The subway is ventilated by means of ex- 
haust fans located in seven fan chambers, some of which con- 
tain two fans, and others only one fan. Each of the fans has a 
capacity of from 30,000 to 37,000 cu. ft. of air per minute, and 
is driven by electric motor, taking current from the trolley 
wires. This system of ventilation has worked satisfactorily. 

Disposal of Raimvater. — The rainwater which enters the 
subway from the inclined entrances, together with that from 
leakage, is lifted from 12 ft. to 18 ft. by automatic electric 
pumps to the city sewers. The subway has pump-wells at the 
l*ablic Garden, at Eliot St., Adams Square, and Haymarket 
Square. In each of these wells are two vertical submerged 
centrifugal pumps made entirely of composition metal. In 
each chamber above, are two electric motors operating the 
pumps. Each motor is started and stopped according to the 
height of water by means of a float and an automatic release 
starting box. The floats are so placed that only one pump 
is usually brought into use. The other, however, comes into 
service in case the first pump is out of order or the water 
enters moi-e rapidly than one pump can dispose of it. In the 
latter case, both motors continue to run until the same low 
level has l)een reached. 

Very little dampness except from atmospheric condensation 
is to be found on the interior walls or roof of the subway, 
althougli numerous discolored patches, caused by dampness and 
dust, may be seen on some parts of the walls. Substantially all 
of the leakage comes through the small drains in the invert 
leading from hollows left in the side walls. Careful measure- 
ment was taken at the end of an unusually wet season to de- 
termine the actual amount of leakage, and the total amount for 
the entire subway was found to be about 81 gallons per minute. 



192 TUNNELING 

Estimated Quantities. — The estimated quantities of material 
used in constructing the subway were as follows : 

Excavation 369,450 cu. yds. 

Concrete 75,660 " " 

Brick 11,105 " " 

Steel 8,105 tons 

Granite 2,285 cu. yds. 

Piles 117,925 lin. ft. 

Ribbed tiles 12,440 sq. yds. 

Plaster 88,190 " 

Waterproofing (asphalt coating) . . . 117,980 " 

Artificial stone . 6,790 " 

Enameled brick 2,210 " 

Enameled tiles 2,855 " 

Cost of the Subway. — The estimated cost of the subway made 
before the work was begun was approximately 14,000,000, and 
the cost of construction did not exceed 13,700,000. This 
includes ventilating and pump chambers, changes of water and 
gas pipes, sewers and other structures, administration, engineer- 
ing, interest on bonds, and all cost whatsoever. Dividing this 
number by the total length we obtain a cost per linear foot of 
$342.30. 

New York Rapid Transit Railway The project of an under- 
ground rapid transit railway to run the entire length of Man- 
hattan Island, was originated some years previous to 1890. In 
1894, however, a Rapid Transit Commission was appointed to 
prepare plans for such a road, and after a large amount of 
trouble and delay this commission awarded the contract for 
construction to Mr. John B. McDonald of New York City, on 
Jan. 15, 1900. Not enough work has been done to enable a 
description of the methods of construction, but the following is 
a brief account of the work to be done : 

Route. — The road starts from a loop which encircles the 
triangular area occupied by the City Hall Park and the Post- 
Office. "Within this loop the tunnel construction will be two- 
storied; but where the main line leaves the loop, all four tracks 



OPEN-CUT TUNNELING METHODS 1931 

will come to the same level, and continue side by side thereafteF 
except at the points which will be noted as the descriptioni 
proceeds. Proceeding from the loop, the four-track line passes- 
under Center and Elm Streets. It will continue under Lafay- 
ette Place, across Astor Place and private property betweeui 
As tor Place and Ninth St. to Fourth Ave. The road will theni 
pass under Fourth and Park avenues until 4 2d St. is reached. 
At this point the line turns west along 4 2d St., which it 
follows to Broadway. It turns northward again under Broad- 
way to the boulevard, crossing the Circle at 59th St. The road 
will then follow the boulevard until 97th St. is reached, where 
the four-track line is separated into two double-track lines. 

At a suitable point north of 96th St. the outside tracks will 
rise so as to permit the inside tracks, on reaching a point near 
103d St., to curve to the right, passing under the north-bound 
track, and to continue thence across and under private property 
to 104th St. From there the two-track tunnel will go under 
104th St. and Central Park to 110th St., near Lenox Ave. ; 
thence under Lenox Ave. to a point near 14 2d St. ; thence 
across and under private property and the intervening streets 
to the Harlem River. The road will pass under the Harlem 
River and across and under private property to 149th St., 
which street it will follow to Third Ave., and will then pass 
under Westchester Ave., where at a convenient point the tracks 
will emerge from the tunnel, and be carried on a viaduct along 
and over Westchester Ave., Southern Boulevard, and Boston 
Road to Bronx Park. This portion of the line, from 96th St. 
to Bronx Park, will be known as the East Side Line. 

From the northern side of 96th St. the outside tracks will 
rise, and after crossing over the inside tracks they will be 
])rought together on a location under the center line of the 
street and proceed along under the boulevard to a point between 
122d and 123(1 streets. At this point the tracks will C(mi- 
nience to emerge from the tunnel, and l)e carried on a viaduct 
along and over the boulevard at a point between 134th and 



194 TUNNELING 

135th streets, where they will again pass into the tunnel under 
and along the boulevard and Eleventh Ave. to a point about 
1,350 ft. north of the center line of 190th St. There the tracks 
will again emerge from the tunnel, and be carried on a viaduct 
across and over private property to Elwood St., and over and 
along Elwood St. to Kingsbridge St. to Kingsbridge Ave., 
private property, the Harlem Ship Canal and Spuyten Duyvil 
Creek, private property, Riverdale Ave., or 230th St. to a ter- 
minus near Bailey Ave. That portion of the line from 96th 
St. to the above mentioned terminus at Bailey Ave. will be 
known as the West Side Line. 

The total length of the rapid transit road, including the 
parts above and below the surface ground of the streets, as well 
as both the East and West Side Lines, will be about 20|^ miles. 

Material Penetrated. — The soil through which the road will 
be excavated, according to numerous borings taken along the 
line, will be a varied one. The lower portion of the road, or 
the part including the loop around the Post-Office up to nearly 
Eourth St., will be undoubtedly excavated through loose soil, 
but from Fourth St. to the ends it will be excavated in rock. 
The loose soil forming the southern part of Manhattan Island 
is chiefly composed of clay, sand, and old rubbish — a soil very 
easy to excavate. There is no fear of any damage to the build- 
ings along the line since, with the exception of the loop around 
the Post-Office, no high buildings are met ; and at the loop the 
underground road passes far above the plane of the foundations 
of the high buildings fronting Park Row. Water will be met 
at some points, but not in such quantities as to be a serious 
inconvenience, except, perhaps, in crossing Canal St., where the 
meeting of a large body of water is expected. From Fourth St. 
to the ends of both the east and west side lines, the soil will be 
chiefly composed, of rock of gneissoid and mica-schistose char- 
acter, these rocks prevailing nearly throughout the whole of 
Manhattan Island. The rock, as a rule, will not be compact, 
but will have seams and fissures, and at many points it will be 



OPEN-CUT TUNNELING METHODS 



195 



found disintegrated and alternated with strata of loose soils, 
and even pockets of quicksand will be met with along the line 
of the road. 

Cross-Sections. — The section of the underground road will 
be of three different types, — the rectangular, the barrel-vault, 
and the circular. The rectangular section. Fig. 113, will be 
used for the greater part of tlie road, of which a portion will 
be for four tracks and a portion for two tracks. The dimensions 
adopted for the four tracks are 50 x 13 ft., and for the double 
tracks 25 x 13 ft. The barrel-vault section, composed of a 




Fig. 113. — Double Track Section, New York Rapid Transit Railway. 

poly centric arch, having the flattest curve at the crown, has been 
adopted for the tunnels under Park Avenue — while tlie semi- 
circular arch is used for all the other portions of the road to be 
tunneled. The circular section of 15-ft. diameter will be used 
under tlie Harlem River, and being for single track, two parallel 
tunnels will be built side by side. 

The main line from the post-office loop to about 102d 
St., consists of four tracks built side by side in one conduit, 
except for that portion under the present Fourth Ave. 
tunnel where two parallel double-track tunnels will be em- 
ployed. The West Side Line will consist of double tracks 



196 TUNNELING 

laid in one conduit, except across Manhattan St. and beyond 
190th St., where it will be carried on an elevated structure. 
The East Side Line will consist of a double-track tunnel driven 
from 10 2d St., and the boulevard under Central Park to 
110th St. and Lenox Ave., and two parallel circular tun- 
nels excavated under the Harlem River, — the other portions 
of the road being double-track, subway and elevated structure. 
The subway, both for four and two tracks, may be built by 
open excavation, cut^and-cover methods. 

For the main line the Slice method, so successfully em- 
ployed in the Boston Subway, will be adopted as the most 
convenient in a case where the width of the excavation is 
great and the traffic enormous, as is the case especially below 
43d St. and along the boulevard. For the double-track sub- 
way, the method of the wide trench will perhaps be adopted 
on account of it being the least expensive ; and since the 
streets where such a trench will be opened are very wide, with 
only a light traffic. 

Lining. — The lining of the subway is of concrete, carried 
by a framework of steel. The floor consists of a foundation 
layer of concrete at least eight inches thick on good founda- 
tion, but thicker, according to conditions, where the founda- 
tion is bad. On top of this is placed another layer of concrete, 
with a layer of waterproofing between the two. In this top 
layer are set the stone pedestals for the steel columns, and the 
members making up the tracks. 

In the four-track subway, the steel framework consists of 
transverse bents of columns, and I-beams spaced about five feet 
apart along the tunnel. The three interior columns of each 
bent are built up bulb angle and plate columns of H-section. 
The wall columns are I-beams, as are also the roof beams ; 
between the I-beams, wall columns, and roof beams there is a 
concrete filling. So that the roof of the subway will be made 
up of concrete arches resting on the flanges of the I-beams of 
the roof. The concrete to be used is of one part Portland 



OPEN-CUT TUNNELING METHODS 



197 



cement, two parts sand, and four parts broken stones. Tlie 
double-track subway will be built in the same way, except that 
only one column is placed between the tracks for the support 
of the roof. 

All the concrete masonry of the roof, foundations, and side 
walls, must contain a layer of waterproofing, so as to keep 
perfectly dry the underground road, and prevent the perco- 
lation of water. This waterproofing must be made up as 
follows • On the lowest stratum of concrete, whose surface 
is made as smooth as possible, a layer of hot asphalt is spread. 
On this asphalt are immediately laid sheets or rolls of felt ; 
another layer of hot asphalt is then spread over the felt, and 





n'o' •-■:„■#+- 



Fig. 114. —Park Avenue Deep Tunnel Construction, New York Rapid Transit Railway. 

then another layer of felt laid, and so on, until no less than 
two, and no more than six, layers of felt are laid, with the felt 
between layers of asphalt. On top of the upper surface of 
asphalt the remainder of the concrete is put in place so as to 
reach the required thickness of the concrete wall. 

Tunnels. — At three points the Standard Subway will be 
replaced by tunnel lines. The location of the three tunnels 
Avill be between 33d and 4 2d St. on Park Ave. ; under 
Central Park, northeast of 104th St., and under the Har- 
lem River. The Park Ave. construction (Fig. 114) will 
consist of two parallel double-track tunnels, located on each 
side of the street, and about 10 ft. below the present tunnel. 
The soil being composed of good rock, the tunnels will be 



198 



TUNNELING 



driven by a wide heading, and one bench, since no strutting 
will be required, and the masonry lining, even of the roof, may 
be left far behind the front of the excavation. The masonry 
lining will consist of concrete walls and brick arches. The 
tunnel under Central Park being driven through a similar 
rock, the same method of excavation and the same manner of 
lining will be used. 

The tunnel under the Harlem River is to be driven through 
soft ground ; and it will be constructed as a submarine tunnel, 
according to the coffer dam process. The tunnels will be 




Fig. 115. — Harlem River Tunnel, New York Rapid Transit Railway. 



lined with iron made up of segments, with radial and cir- 
cumferential flanges. Concrete will be placed inside and flush 
with the flanges. 

The tracks, both in the subway and tunnels, are an inti- 
mate part of the concrete flooring. The rail rests on a con- 
tinuous bearing of wooden blocks, laid with the grain running 
transversely with respect to the line of the rail, and held in 
place by two channel iron guard rails. The guard rails are 
bolted to metal cross-ties embedded in the concrete. 



OPEX-CUT TUNNELING METHODS 199 

Viaduct. — A considerable portion of the double track branch 
lines north of 103d St. will be viaduct, or elevated structure. 
The viaduct construction on the West Side Line will extend, in- 
cluding approaches, from 122d St. to very near 135th St. 
Of this distance, 2,018 ft. 8 ins. will be viaduct proper, 
consisting of plate girder spans carried by trestle bents at the 
ends, and by trestle towers for the central portion. The 
columns of the bents and towers are to be built up bulb-angle 
and plate columns of H-section of the same form as those used 
in the bents inside the subway. The approaches proper will be 
built of masonry. The elevated line proper consists of plate 
girder spans, supported on plate girder plate cross girders 
carried by columns. 

Stations. — Many stations will be built along the line. 
These will be located on each side of the street. The 
entrances at the stations will consist of iron framework, with 
glass roofs covering the descending stairways. The passage- 
ways leading down will be walled with white enameled bricks 
and wainscoted with slabs of marble. The stations for the 
local trains will be located on each side of the road close to the 
walls, since the outside tracks are reserved for the local trains, 
while the middle ones will be reserved for the expresses. The 
few stations for the express trains will be located between the 
middle and outside tracks. Stations will be provided with all 
the conveniences required, having toilet rooms, news stands, 
benches, etc., and will be lighted day and night by numerous 
arc lamps. 

General. — The contractor is compelled to complete the 
work in four and one-half years, but he has promised to have 
it in full running order within three years. There is no diffi- 
culty in doing this, since the great extension of the road and 
the great width of the avenues under which it runs allow work 
all along the line at the same time. The work, briefly summar- 
ized, comprises the following items: — 



200 TUNNELING 

Length of all sections, ft 109,570 

Total excavation of earth, cu. yds 1,700,228 

Earth to be filled back, " 773,093 

Rock excavated, " 921,128 

Rock tunneled, " 368,606 

Steel used in structure, tons 65,044 

Cast iron used, " 7,901 

Concrete, cu. yds 489,122 

Brick, " 18,519 

Waterproofing, sq. yds 775,795 

Vault lights, " 6,640 

Local stations, number .... : 43 

Express stations, " 5 

Station elevators, " 10 

Track total, lin. ft 305,380 

" underground, lin. ft 245,514 

" elevated, " 59,766 

In addition to the construction of the railway itself, it will 
be necessary to construct or reconstruct certain sewers, and to 
adjust, readjust, and maintain street railway lines, water pipes, 
subways, and other surface and subsurface structures, and to 
relay street pavements. 

The total cost of the work, according to the contract signed 
by Mr. McDonald, will be 135,000,000. Dividing this amount 
by the total length of the road, which is 109,570 lineal feet, 
we have the unit cost a lineal foot $315, or a little less than 
unit of cost of the Boston subway, which was $342 per 
lineal foot. 

The road belongs to the city. The contractor acts as an 
agent for the city in carrying out the work, and he is the leaser 
of the road for fifty years. The work is paid for as soon as 
the various parts of the road are completed, and the money is 
obtained from an issue of city bonds. During the fifty years' 
lease the contractor will pay the interest plus 1 % of the face 
Talue of the bonds. This 1 % goes to the sinking-fund, which 
within the fifty years at compound interest forms the total sum 
required for the redemption of bonds. 



SUBMARINE TUNNELING 201 



CHAPTER XVIII. 

SUBMARINE TUNNELING: GENERAL DISCUS- 
SIGN. — THE SEVERN TUNNEL. 



GENERAL DISCUSSION. 

Submarine tunnels, or tunnels excavated under the beds of 
rivers, lakes, etc., have been constructed in large numbers 
during the last quarter of a century, and the projects for such 
tunnels, which have not yet been carried to completion, are 
still more numerous. Among the more notable completed 
works of this character may be noted the tunnel under the 
River Severn and those under the River Thames in England, 
the one under the River Seine in France, that under the 
St. Clair River for railway, that under the East River for gas 
mains, that under Dorchester Bay, Boston, for sewage, and 
those under Lakes Michigan and Erie for the water supply of 
Chicago and Cleveland in America. Among the partly com- 
pleted submarine tunnels which have been abandoned the most 
notable example is, perhaps, the Hudson River tunnel. For 
the details of the various projected submarine tunnels of note, 
which include tunnels under the English and Irish Channels, 
under the Straits of Gibraltar, under the sound between 
Copenhagen in Denmark and Malino in Sweden, under the 
Messina Straits between Italy and Sicily, and under the Straits 
of Northumberland between New Brunswick and Prince 
Edward Island, the reader is referred to the periodical litera- 
ture of the last few years. 

Previous to attempting the driving of a submarine tunnel 
it is necessary to ascertain the character of the material it will 



202 TUNNELING 

penetrate. This fact is generally determined by making dia- 
mond-drill borings along the line, and the object of ascertaining 
it is to determine the method of excavation to be adopted. If 
the material is permeable and the tunnel must pass at a small 
depth below the river bed, a method will have to be adopted 
which provides for handling the difficulty of inflowing water. 
If, on the other hand, the tunnel passes through impermeable 
material at a great depth, there will be no unusual trouble 
from water, and almost any of the ordinary methods of tun- 
neling such materials may be employed. Shallow submarine 
tunnels through permeable material are usually driven by the 
shield method or by the compressed air method, or by a method 
which is a combination of the first and second. 

It is not an uncommon experience for a submarine tunnel 
to start out in firm soil and unexpectedly to find that this 
material becomes soft and treacherous as the work proceeds, or 
that it is intersected by strata of soft material. The method of 
dealing with this condition will vary with the circumstances, but 
generally if any considerable amount of soft material has to be 
penetrated, or if the inflow of water is very large, the firm- 
ground system of work is changed to one of the methods 
employed for excavating soft-ground submarine tunnels. The 
Milwaukee water supply tunnel and the East River gas tunnel, 
described elsewhere, are notable examples of submarine tunnels 
began in firm material which unexpectedly developed a treacher- 
ous character after the work had proceeded some distance. 
Occasionally the task of building a submarine tunnel in the 
river bed arises. In such cases the tunnel is usually built by 
means of cofferdams in shallow water, and by means of caissons 
in deep water. 

Submarine tunnels under rivers are usually built with a de- 
scending grade from each end which terminates in a level middle 
position, the longitudinal profile of the tunnel corresponding to 
the transverse profile of the river bottom. Where, however, 
such tunnels pass under the water with one end submerged, and 



SUBMARINE TUNNELING 203 

the other end rising to land Uke the water supply tunnels of 
Chicago, Milwaukee, and Cleveland, the longitudinal profile is 
commonly level, or else descends from the shore to a level 
position reaching out under the water. 

The drainasfs of submarine tunnels durino^ construction is 
one of the most serious problems with which the engineer has 
to deal in such works. This arises from the fact that, since the 
entrances of the tunnel are higher than the other parts, all of 
the seepage water remains in the tunnel unless pumped out, and 
from the possibility of encountering faults or permeable strata, 
which reach to the stream bed and give access to water in 
greater or less quantities. Generally, therefore, the excavation 
is conducted in such a manner that the inflowing water is led 
directly to sumps. To drain these sumps pumping stations 
are necessary at the shore shafts, and they should have ample 
capacity to handle the ordinary amount of seepage, and enough 
surplus capacity to meet probable increases in the inflow. For 
extraordinary emergencies this plant may have to be greatly 
enlarged, but it is not usual to provide for these at the outset 
unless their likelihood is obvious from the start. The character 
and size of the pumping plants used in constructing a number 
of well-known tunnels are described in Chapter XII. 

In this book submarine tunnels will be classified as follows: 
(1) Tunnels in rock or very compact soils, which are driven by 
any of the ordinary methods of tunneling similar materials on 
land; (2) tunnels in loose soils, which may be driven, (a) by 
compressed air, (6) by shields, or (c-) by shields and compressed 
air combined ; (3) tunnels on the river bed, which are con- 
structed, (a) by cofferdams, or (6) by caissons ; (4) tunnels 
partly in firm soil and partly in treacherous soils, which are 
driven partly by one of the firm-soil methods, and partly by one 
of tlie soft-soil methods. To illustrate tunnels of the first class, 
the River Severn tunnel in England has been selected ; to 
illustrate those of the second class, the several tunnels discussed 
in the chapter on the shield system of tunneling and the ^lil- 



204 tun:nelikg 

waukee tunnel is sufficient ; to illustrate those of the third class, 
the Yan Buren Street tunnel in Chicago is selected ; and to 
illustrate those of the fourth class, the East River gas tunnel 
and the Milwaukee water supply tunnels are excellent examples. 

THE SEVERN TUNNEL. 

The Severn tunnel, which carries the Great Western Rail- 
way, of England, beneath the estuary of a large river, is 4 miles 
642 yards long. It penetrates strata of conglomerate, limestone, 
carboniferous beds, marl, gravel, and sand, at a minimum depth 
of 44f ft, below the deepest portion of the estuary bed. The 
following description of the work is abstracted from a paper by 
Mr. L. F. Yernon-Harcourt. * 

The first work was the sinking of a shaft, 15 ft. in diameter, 
lined with brickwork, on the Monmouthshire bank of the Severn, 
200 ft. deep. After the Monmouthshire shaft had been sunk, a 
heading 7 ft. square was driven under the river, rising with a 
gradient of 1 in 500 from the shaft on the Monmouthshire shore, 
so as to drain the lowest part of the tunnel. Near to the first, a 
second shaft was sunk and tubbed with iron, in which the 
pumps were placed for removing the water from the tunnel 
works, connection being made by a cross-heading with the 
heading from the first shaft. There was also a shaft on the 
Gloucestershire shore ; and two shafts inland from the first on 
the Monmouthshire side, to expedite the construction of the 
tunnel. Headings were then driven in both directions along the 
line of the tunnel, from the four shafts ; and the drainage head- 
ing from the old shaft was continued, in the line of the tunnel, 
under the deep channel of the estuary, and up the ascending 
gradient towards the Gloucestershire shore, till, in October, 
1879, it had reached to within about 130 yards of the end of 
the descending heading from the Gloucestershire shaft. During 
this period, though the work had progressed slowly, no large 

* Proceedings Inst. C.E., vol. cxxi. 



SUBMARINE TUNNELING 205 

quantity of water had been met with in any of the headings, in 
spite of their already extending under ahnost the whole width 
of the estuary. On October 18, 1889, however, a great spring 
was tapped by the heading which was being driven landwards 
from the old shaft, about 40 ft. above the level of the drainage 
heading ; and the water poured out from this land spring in 
such quantity that, flowing along the heading, falling down the 
old shaft, and thus finding its way into the drainage heading 
and the long heading of the tunnel under the estuary in con- 
nection with it, it flooded the whole of the workings in com- 
munication with the old shaft, which it also filled within twenty- 
four hours from the piercing of the spring. 

To obtain increased security against any influx of water 
from the deep channel of the estuary into the tunnel, the 
proposed level portion of the tunnel, rather more than a 
furlong long under this part, was lowered 15 ft. by increas- 
ing the descending gradient on the Monmouthshire side from 
1 in 100 to 1 in 90, and lowering the proposed rail level on 
the Gloucestershire side 15 ft. throughout the ascent, so as not 
to increase the gradient of 1 in 100 against the load. A 
new shaft, 18 ft. in diameter, was sunk slightly nearer the 
estuary on the ^Monmouthshire shore than the old one ; two 
shafts also were sunk on the land side of the great spring for 
pumping purposes ; and additional pumping machinery was 
erected. The flow from the spring mto the old shaft was 
arrested by a shield of oak fixed across the heading ; and 
at last, after numerous failures and breakdowns of the pumps, 
the headings were cleared of water, after a diver, supplied with 
a knapsack of compressed oxygen, had closed a door in the 
long heading under the estuary ; and the works were resumed 
nearly fourteen months after the flooding occurred. The great 
spring was then shut off fi-om the workings by a wall across 
the heading leading to the old shaft; and, owing to the lower- 
ing of the level of the tunnel, a new drainage heading had to 
be driven from the bottom of the new shaft at a lower level, 



206 TUNNELING 

which was made 5 ft. in diameter, and lined with brickwork, 
whilst the old drainage heading was enlarged to 9 ft. in diam- 
eter, and lined with brickwork, so as to aid in the permanent 
ventilation of the tunnel. The lowering of the level, moreover, 
converted the bottom tunnel headings into top headings, so 
that along more than a mile of the tunnel the semicircular arch, 
26 ft. in diameter, was built first, and then, after lowering the 
headings, the invert was laid and the side walls were built up. 
Bottom headings were driven along the remainder of the tunnel, 
and the work was expedited by means of break-ups. Ventila- 
tion was effected in the works by a fan 18 inches in diameter 
and 7 ft. wide, fixed at the top of the new deep shaft ; the rock 
was bored by drills worked by compressed air ; the blasting was 
eventually effected exclusively by tonite, owing to its being 
freer from deleterious fumes than any other explosive ; and the 
workings were lighted by Swan and Brush electric lamps. The 
tunnel is lined throughout with vitrified brickwork, between 
2i ft. to 3 ft. thick, set in cement, and has an invert 1| ft. to 
3 ft. in thickness ; the lining was commenced towards the end 
of 1880, the headings under the river were joined in Septem- 
ber, 1881, and the last length of the tunnel, across the line of 
the great spring, was completed in April, 1885. 

Water came in from the river through a hole in a pool of 
the estuary, close to the Gloucestershire shore, in April, 1881, 
during the lining of a portion of the tunnel, but fortunately 
before the headings were joined. This influx was stopped by 
allowing the water to rise in the tunnel to tide-level, to prevent 
the enlargement of the hole, which was then filled up at low 
water with clay, weighted on the top with clay in bags. The 
great spring broke out again in October, 1883, and flooded the 
works a second time ; but within four weeks the water had 
been pumped out and the spring again imprisoned. During 
this period an exceptionally high tide, raised still higher by 
a southwesterly gale, inundated the low-lying land on the Mon- 
mouthshire side of the estuary, and, flowing down one of the 



SUBMARINE TUNNELING 207 

inland shafts, flooded a section of the tunnel, but the pumps 
removed this water within a week. 

In order to construct ths portion of tunnel traversing the 
line of the great spring, the water was diverted into a side 
heading below the level of the tunnel, leading to the old shaft, 
whence it was pumped, and the fissure below the tunnel was 
filled with concrete, over which the invert was built. An 
attempt to imprison the spring, on the completion of this 
length of tunnel, having resulted in imposing an excessive pres- 
sure on the brickwork, leading to fractures and leakage, a shaft, 
29 ft. in diameter, was sunk at the side of the tunnel at this 
point in 1886, and pumps were erected powerful enough to 
deal with the entire flow of the spring. 

The tunnel was opened for traffic in December, 1886, and 
gives access to a double line of railway, connecting the lines 
converging to Bristol with the South Wales railway and the 
western lines. The pumping power provided at the shaft con- 
nected with the great spring, and at four other shafts, is capable 
of raising 66,000,000 gallons of water per day, the maximum 
amount pumped from the tunnel being 30,000,000 gallons a 
day. The ventilation of the tunnel is effected by fans placed 
in the two main shafts on each bank of the estuary, and the fan 
in the Monmouthshire shaft is 40 ft. in diameter, and 12 ft. 
wide. The tunnel gives passage to a large traffic, numerous 
through-trains between the north and southwest of England 
making use of it. 



208 TUNNELING 



CHAPTER XIX. 

SUBMARINE TUNNELING (Continued) ; THE EAST 

RIVER GAS TUNNEL. — VAN BUREN ST. 

TUNNEL, CHICAGO. 



The East River gas tunnel is a notable example of a tunnel 
begun in firm soil which unexpectedly developed treacherous 
strata. It is also remarkable from the fact that the shield which 
was employed to overcome the trouble was driven from rock 
into soft material and from the soft material into rock again 
with the utmost success. The following description of the 
work is abstracted from a paper by Mr. Walton I. Aims, the 
engineer in charge of the work, published in the Journal of 
the Association of Engineering Societies for May, 1895, and in 
Engineering News of July 11, 1895. The accompanying cuts 
are reproduced from the last-named publication. 

During 1891 and 1892 the East River Gas Co., of Long 
Island City, a corporation with works situated on the Long 
Island shore of the East River, obtained from the New York 
State Legislature a new charter, and such necessary legislation 
as to permit the extension of their mains across the East River 
into the city of New York. 

The feasibility of constructing a tunnel under the river 
through which the gas mains might be laid was discussed, and 
after some preliminary surveys and examinations a route was 
decided upon from the works of the company at Ravenswood, 
Long Island City, to between 70th and 71st Sts., New York, 
passing under Blackwell's Island and the east and west chan- 
nels of the East River. On about this line of location some 
eight or ten pipe soundings were made in the two river chan- 



iSlli.MAKlNE TUNNELING 209 

iiels, all of which indicated a rock bottom ; and the results of 
these, together with surface indications, where at both the Long 
Island and New York shores, as well as on Blackwell's Island, 
bedrock lay exposed, led all to conclude that nothing but rock 
was to be encountered. On these investigations a contract was 
entered into on June 25, 1891, for the construction of a su|>- 
posedly rock tunnel, which the contractor guaranteed to com- 
plete by April, 1893. 

Work was begun at the Ravenswood or Long Island side 
on June 28 by sinking a shaft 9 ft. square about 200 ft. back 
from the river to a depth of about 148 ft. below the surface ; 
while at New York, on July 7, a shaft of the same dimensions 
was sunk to a depth from the surface of 139 ft. In both these 
shafts rock was entered after about 8 ft. of soil ; but while the 
rock at New York was quite dry, at Ravenswood it proved 
seamy and very wet. 

The tunnel-roof grade had been established at 109 ft. below 
mean high water at the New York shaft, with a grade for drain- 
age of ^ % towards Ravenswood. This gave a minimum cover 
of 41 ft. at the deepest point in the west or New York channel 
on the East River, where there is 70 ft. of water at mean high 
tide. The east or Long Island channel is comparatively shallow, 
the deepest point being only 35 ft. below mean high water level. 
The one thing feared was that fissures yielding large volumes 
of water might extend to the tunnel roof and largely augment 
the cost of pumping. The size of the tunnel section was to be 
8 ft. 6 ins. in height by 10 ft. 6 ins. in width, this giving sufB- 
cient room for the laying of two 3-ft. gas mains and one 4-ft. 
main. 

In the shafts, on both sides of the river, the headings were 
now turned. At Ravenswood the work was delayed by meet- 
ing considemble quantities of silty water, but at New York the 
tunnel was practically dry until towards the end of December, 
1892, when, at a distance of 338 ft. from the shaft, a fissure 
was struck yielding about a 3-in. stream of salt water. The 



210 TUNNELING 

rock to within 20 ft. of this point had been the regular hard 
New York gneiss, with a dip towards Long Island of 10° from 
the vertical, and a strike north and south at right angles to the 
direction of the tunnel. Here it gradually began to soften, 
becoming more and more micaceous until when about 20 ft. 
beyond the water-bearing fissure the rock suddenly terminated, 
running into a vein of soft material with the same dip and 
strike as that of the rock. 

This new material proved to be a vein, principally of decom- 
posed feldspar, gray in color, crumbling easily, and with no 
perceptible grit. It still preserved a rock structure, and was 
perfectly dry when undisturbed. But its exposed surfaces 
were quickly acted upon by water, which it would absorb and 
then wash away quite rapidly. The water-bearing fissure and 
this soft vein were connected ; more water was also met at the 
junction of the rock and the soft material, and later experience 
proved that in passing through these soft veins water was 
always to be found next to the rock — a sort of water-course 
on both sides of the soft vein. Had it not been for encounter- 
ing this water, the tunnel might have been carried through the 
soft material without employing compressed air, though the 
prudence of attempting this might be questioned, for nothing 
more insures the safety of both the men and the work than 
compressed air in sub-aqueous tunneling. 

The finding of this soft material, so unexpected, was quite 
a set-back to all concerned. However, it was decided to drive 
a small timbered drift about 4 ft. wide by 6 ft. high to investi- 
gate the ground ahead, and find how much of this material was 
to be penetrated before solid rock was again met. This drift 
was started and driven in for about 6 ft. Meanwhile a most 
destructive action was going on between the water and the soft 
material. The water running along the face of the rock had 
washed out a cavity overhead in the soft ground. The walls 
of this cavity were gradually breaking away, and the clay-like 
substance falling down would close the outlet of the water into 



SUBMARINE TUNNELING 211 

the tunnel. The water would then accumulate in this pocket, 
softening up fresh material on the sides until it had gained a 
sufficient head to burst through the dam which confined it, 
when it would come rushing into the tunnel, carrying with it 
large quantities of the softened material. These rushes were 
accompanied by a loud bubbling sound that quite mystified the 
men, which was, of course, the sound of the air displacing the 
water in the cavity. As soon as the pocket had emptied itself, 
for a time the trouble was over, until with the falling of more 
material the outlet was again closed and the operation was 
repeated. These rushes of water, with the accompanying 
sound of the bubbling air, soon became more and more alarming 
to the men. The cavity was constantly increasing in size, and 
extending up toward the river-bed. Each recurrence would 
now send the men running for the shaft, by no means certain 
that the river had not at last made a connection wit?h the 
tunnel. 

All work in the small drift was abandoned, and on Dec. 31 
a bulkhead was hurriedly constructed at the face to prevent the 
threatened flooding of the shaft. Up to this time over 25 yards 
of material had been washed into the tunnel, all of which had 
come from along the rock face. With the river-bed only 45 ft. 
above the tunnel-roof, there is every reason to believe that this 
bulkhead was put in none too soon, and a connection with the 
river narrowly averted. The bulkhead was well packed with 
hay to prevent, as much as possible, further washing of the 
material, and a discussion was now entered into as to the 
method of future procedure. The contractors were in favor of 
abandoning the heading and returning to the shaft, to sink to a 
lower level and start anew in hopes of meeting more favorable 
conditions at a greater depth. There had been a somewhat 
similar experience on the Croton Aqueduct, where that tunnel 
passes under the Harlem River. Soft material had been en- 
countered on the first established level, which proved so trouble- 
some that after two or three unsuccessful attempts had been 



212 TUNNELING 

made to pass through it, it was finally decided to abandon the 
heading and return to the shaft, sinking some 150 ft. deeper. 
On this new level nothing but rock was encountered. In the 
East River tunnel, however, the soft material was clearlj a 
decomposed vein, and to what depth this decomposition might 
extend was unknown ; so that as there were no well-founded 
reasons, in this case, for expecting any better conditions at a 
lower level, it was decided to first attempt to drive the present 
heading, in compressed air, leaving the sinking as a later ex- 
pedient should the proposed means fail. An arrangement was 
made with the contractors by which the company was to share 
the expense of the work in soft ground. 

It was at this time that the writer became connected with 
the work, having charge of installing and conducting the com- 
pressed air operations for the company. To form the com- 
pressed air-working chamber, a solid brick wall or bulkhead 
8 ft. thick was built across the tunnel into gains in the rock 
about 40 ft. back from the heading, and containing a cylindrical 
steel air-lock 6 ft. in diameter and 10 ft. long. 

In the engine room, the 18 x 24-in. IngersoU piston-inlet 
compressor, used heretofore for running the rock-drills, was 
supplemented by a small Rand compressor, and both arranged 
to supply, independently, compressed air to the working cham- 
ber below. Incandescent electric lighting was introduced into 
the tunnel, which is almost a necessity in compressed air opera- 
tions, as common illuminants produce an enormous quantity of 
smoke when burning in compressed air. A telephone was also 
taken into the working chamber, by which instant communica- 
tion could be had with the engine room in case any sudden 
increase of air pressure should be desired. 

On Feb. 25, 1893, operations were commenced, in the 
heading, under 35 lbs. of air pressure. The previous work 
here had greatly increased the difficulties, and it was not long 
before the air pressure had to be raised to 42 lbs. to control 
the water. The excavation was advanced under a cylindrical 



SUBMAKINE TUNNELING 213 

steel roof, built up of plates 3 ft. long and 1 ft. wide, of |-in. 
sheet steel, to the four sides of which were riveted angle bars 
2|- X 2^ X :^ in. These plates were bolted together in a heading 
about 6 ft. high. In the erection of this roof, poling-boards 
were used for each plate, and a bulkhead carried down with 
each ring as erected. When the heading had been advanced 
about 20 ft. from the rock, a 12 x 12 in. yellow-pine mudsill 
was introduced along the bottom of the heading, and on this 
the roof was covered by means of radial timber bracing. The 
excavation was now carried down on both sides of this mudsill, 
to a distance of about 10 ft. from the rock, the steel roof being 
extended well down on the sides. A circular section was thus 
excavated, in which brickwork was laid, four courses thick, and 
with an internal diameter of 10 ft. Between March 4 and 6 
a great deal of trouble Avas experienced. Air pressure was 
several times to 48 lbs., and the work progressed very slowly 
on account of the many inrushes of water and softened mate- 
rial. It was not until April 8 that the last section of brickwork 
in the soft material was completed and rock again entered, after 
passing through 29 ft. of this decomposed material. Of the 
material met in driving through this vein, at first 9 ft. of the 
gray decomposed feldspar was penetrated, a vein of 4 ins. of 
hard quartz was then met, and this was followed by 6 ft. of pure 
white decomposed feldspar, smooth and soft as plaster. The 
remaining 14 ft. was made up of layers of feldspar and chk)rite. 
This chlorite, deep green in color, flaky and grease-like to the 
touch when wet, proved to be very troublesome material, as it 
was easily converted into a fluid state by the water, whicli Avas 
again encountered next to the rock. 

At the Long Island shaft, the work up to this time had pro- 
gressed to about 250 ft. from the shaft. The material so far 
encountered on this side was a hard, seamy gneiss, bearing con- 
siderable quantities of salty water, containing iron, lime, and 
magnesia. Soft ground was nov/ met at this end, in a seam 
about 4 ft. wide, of chlorite. As this material Avas perfectly 



214 TUNNELING 

dry and not thoroughly disintegrated, the tunnel was timbered 
through this seam without difficulty. Several similar veins 
were thus met and passed through, until at a point 285 ft. from 
the shaft, where after drilling for about 2 ft. through rock a soft 
green, almost liquid chlorite vein was struck, which began flowing 
in through the drill holes with great force. These holes were 
plugged ; but as it was necessary to know what was ahead, and 
as with 100 ft. of cover between the tunnel roof and the river 
bottom it was thought that the condition of affairs could not be 
very serious, it was decided to continue driving ahead without 
air pressure, and with a timbered heading. To see what the 
material would do, several hand-holes were put into the rock- 
face with the object of blasting out a hole about 2 ft. square 
through the remaining 2 ft. of rock, to the chlorite. Before 
blasting, however, the precaution was taken to build a bulk- 
head, some 40 ft. back from the face. On firing the holes an 
inrush of many yards of material took place, which was finally 
checked by some rock fragments closing the opening through 
the rock. After several desperate attempts on the part of the 
contractors to control this material and make progress, the work 
was finally abandoned in the latter part of March, and as a 
4-in. stream of water was now flowing from the heading, pump- 
ing was discontinued, and the shaft and tunnel allowed to flood. 
At the New York end work was still being carried on in 
compressed air. The rock encountered at the other side of the 
soft seam closely resembled the decomposed material which had 
been penetrated before, and consisted of alternate layers of 
feldspar and chlorite, with an occasional vein of quartz. It 
was quite soft, though requiring drilling and blasting, and 
eventually it had to be lined. After the heading had been 
driven about 69 ft. into this rock the company decided, in 
spite of the uncertainty as to the material ahead, to remove the 
air pressure, and to call upon the contractors to resume their 
contract. Upon removing the air pressure, however, the brick- 
work through the soft seam proved so unsatisfactory in exclud- 



SUBMARINE TUNNELING 



215 



ing the water that air pressure was again put on, and it was 
decided to line the brickwork with a circular cast-iron lining 
(Fig. 116). Although this brickwork was only 10 ft. in in- 
side diameter, a lining was designed 10 ft. 2 ins. in the clear, 
as it was now desired to make the tunnel bore as large as 
possible. To put in this lining, some of the brickwork had to 
be cut out, which was then removed in sections, enough for 
one ring of plates at a time. The lining consisted of rings 

of plates or segments, 
'i^;??^^^'l,<?t^st^ns^,^ ^^pf^ each segment being 

about 3 ft. long and 
1 ft. 4 ins. wide, with 
internal flanges 4 ins. 
deep, from the back of 
the plate. The metal 
in both the back of 
the plate and the 
flanges was 1^ ins. 
thick. All the joint- 
faces of the segments 
were planed, and 1-in. 
bolts used for fasten- 
ing them together. A 
complete tunnel ring 
was composed of nine segments and a small inverted key, 
about 8 ins. wide. 

Difliculties between the company and the contractors, which 
had been brewing for some time, now culminated and the 
courts were appealed to, to settle their differences. 'Jliis 
caused a cessation of work for a short time until tlie com- 
pany were empowered to take possession and resume the work 
of construction for themselves. The work of putting the cast- 
iron lining into the brickwork was necessarily a very slow 
operation. The lining was extended well into the rock on 
both sides of the soft vein, and a wall built at both ends, be- 




Cross Section. 



Fig. 116. — Sections of Cast Iron Lining, East River 
Gas Tunnel. 



216 TUNNELING 

tween the rock and the iron lining, to confine the Portland 
cement grout, which was now introduced back of the plates. 
To effect this grouting 1^-in. holes had been drilled and tapped 
through the back of several plates in each ring. Through 
these holes the grout was pumped by means of a Cameron 
pump ; and after the space between the brickwork and the 
lining had been thoroughly grouted, the work was found, on 
taking off the air pressure from the heading, to be perfectly 
water-tight. It was not until towards the end of July that the 
work of lining the brickwork was completed and driving ahead 
in the rock was resumed. Then, when an advance of only 10 
ft. had been made, a second soft seam was encountered about 
■80 ft. beyond the first one, and a test pipe was driven to a 
liorizontal depth of 70 ft., without encountering anything 
solid. To avoid further delay, the driving of the test-pipe 
was discontinued at this depth, and preparations made for 
advancing the heading. For this test-pipe 1^-in. common 
wrought-iron pipe was used, which was driven in by a small 
machine-drill, and washed out at each lengthening of the 
pipe with a l^-in. wash-pipe. P>om these washings the differ- 
ent materials penetrated were sampled, with the following 
tabulated results : 

3 ft. gray decomposed feldspar and chlorite. 

11 ft. soft black mud, containing lumps of carbonized wood like charcoal. 
19 ft. hard black mud and sand, with nodules of pyrites. 
22 ft. gray decomposed feldspar. 

4 ft. decomposed feldspar and chlorite. 
11 ft. gray decomposed feldspar. 

Water was again found next to the rock, but was consider- 
ably held in check by the compressed air. As from the results 
of the test-pipe there were no special difficulties to apprehend 
from the indicated material, it was decided to drive ahead, 
under the open heading method, as this involved no delays in 
waiting for special machinery. The light steel cylindrical roof 
was again used in advancing the excavation, but for the perma- 



SUBMARINE TUNNELING 217 

iieiit lining the cast-iron rings were to be introduced instead 
of brickwork, as heretofore. A start was made on Aug. 7 to 
drive the heading into the soft material, but two days later, after 
the work had been advanced 6 ft. into the soft vein, ordeis 
were received to suspend all work on account of the great 
financial depression of the time. This was unfoitunate ; and 
could it have been anticipated a few days the heading into the 
soft material would have been left unopened. As it Avas now, 
from being first disturbed and then abandoned, the water was 
first allowed to soften up the black mud in the heading, and, in 
spite of the bulkhead, a considerable quantity of the material 
was washed into the tunnel. 

This stay of proceedings was utilized by making a horizon- 
tal test boring in the heading on the Long Island side. At 
this sliaft no work had been done since the departure of the 
contractors, beyond the building of a brick l)ulkhead and air- 
lock in the tunnel. Compressed air had tlien been put on, 
which considerably reduced the amount of water flowing into 
the tunnel from the heading. The action of the compressed 
air had been somewhat pecnliar; for notwithstanding the great 
depth of the tunnel below the river bed, at 10 lbs. pressure the 
air began to escape through the heading, and with a pressure 
of 35 lbs. per sq. in. small bubbles of escaping air could be 
seen rising to the surface for over 300 ft. up and down the 
river. This seemed to indicate that the ground above the 
tunnel had been honeycombed up to the river bottom by 
the previous washing-in of such quantities of the soft green 
chlorite. As it wjis known that there were detached lumps of 
rocks in this soft vein, 2-in. heavy pipe was used for the test 
boring, with drive-well c()Ui)lings, and a circulai-, hollow steel 
bit for the cutting end. This pipe was driven in the same way 
as the one on the New York side, and after passing through 
chlorite and various kinds of soft-rock fragments, solid rock 
was again met at 32 ft. Into this rock a hole was drilled to i) 
depth of ')4 ft., using a small bit on the end of a 1-in. pipe aiK^ 



218 



TUNNELING 



drilling through the test-pipe. The rock beyond the soft 
seam was a soft white limestone. 

With the prospect of resuming work the question now 
arose as to the best method of proceeding; and, as a great 
deal depended upon the success of driving through the present 
headings, it was strongly recommended that the safest and 
surest method, that of shield tunneling, be adopted in both 
headings, although necessarily entailing a large expenditure in 
plant, and delay in time for installation. This plan met with 







Longrrudinat Section. 



End View-of HeaA. 



Fig. 117. — Section and Elevation of Shield, East River Gas Tunnel. 



the company's approval, and a shield and hydraulic plant were 
designed. As the nature of the material to be penetrated be- 
yond the test-pipes was unknown, this shield was so made that 
in passing from rock to soft material, or back again to rock, it 
could be erected or taken apart again with a minimum of time 
and labor, so that it might almost be called a portable shield 
(Fig. 117). As in both the tunnel headings there was but one 
air-lock, and as it was inadvisable to remove the air pressure 
from the headings, the different parts of the shield had to be of 
such size as could be passed through the air-lock doors. This 



SUBMARINE TUNNELING 



219 



was accomplished by dividing tlie shield transversely, separat- 
ing the tail-end section, or that which overlaps the tunnel, from 
the cutting-edge section containing the working chambers. 
These two sections were, of course, circular, 11 ft. | in. out- 
side diameter. The tail end section was 3 ft. 6 ins. long, ar.d 
the cutting-edge section 3 ft. 8 ins. long. Both of tl cpc 
sections were again divided, longitudinally, into four quadra] Is. 
The outside shell, in both tail-end and cutting-edge sectioi:^, 
was made up of one h i^^- ^^^^ <^i^6 « i^^- steel plates riveted 
together ; and at the four quadrant joints, there were -^-in. butt- 
straps 12 ins. wide running the whole 
length of the shield and uniting the 
quadrants and the two sections. The 
middle diaphragm, separating the 
cutting-edge and tail-end sections, 
was made of two plates, one riveted 
to each of the two sections, and these 
two plates bolted together with the 
butt-straps united the sections. The 
cutting-edge section contained two 
platforms, one vertical and one hori- 
zontal, of the same length as the 
section. 

To erect this shield the only rivet- 
ing necessary was at the four butt- 
strap joints in the tail-end section, where it was necessary to 
preserve a flush surface on both sides of the outer shell. In 
the cutting-edge part countersunk bolts were used through the 
butt-straps. About 380 ^-in. bolts and 160 rivets were used to 
erect the shield. Two doors closing each of the four working 
chambers were liung on the vertical platform, and were pro- 
vided with fastenings so that the whole face could be easily 
closed. 

To drive the shield 12 o-in. hydraulic jacks were used, 
designed for a working pressure of 5,000 lbs. per sq. in., or 




Fio. 118. — Elevation and Section 
of Hydraulic Jack, East River 
Gas Tunnel. 



220 TUNNELING 

700 tons on the whole shield (Fig. 118). These jacks were 
controlled by two block-valves, one placed on each side of 
the shield. Each of these block-valves consisted of six inde- 
pendent valves all in one compact casting, each of which had a 
pressure and exhaust stem. Half-inch XX pipe was used for 
connecting each jack with its valve, and 1-in. hydraulic pipe 
was used for the pressure main, which was connected with the 
shield block-valves by three swivel-joint connections. To fur- 
nish the pressure, a very compact little pump, designed by 
Watson & Stillman, of New York, was used without an accu- 
mulator, the pressure being very nicely governed by a steam- 
regulating valve. 

On Sept. 22 work was resumed on the New York side, 
with a small force of men working days only, to excavate in 
the rock an enlarged chamber about 15 ft. back from the face, 
in which to erect the shield. This chamber was made circular, 
about 15 ft. in diameter and 10 ft. long. Back from this, the 
rock was taken out in a circular form of about 11 ft. diameter, 
for some 14 ft., or enough for about 10 rings of the cast-iron 
segments which were here erected in the rock, the spaces be 
tween being thoroughly grouted with Portland cement. These 
rings were thus made solid in the rock to withstand the thrust 
of the shield-jacks upon the lining. The blasting necessary in 
this work was made as light as possible ; but it was not without 
its effect upon the soft material in the heading, a considerable 
quantity of the black mud being washed through the bulkhead, 
while the braces showed signs of a heavy strain from the 
squeezing of the material. The shield arrived at the works on 
Nov. 10, and the work of erection was immediately begun. 
The sections were lowered down the shaft and taken through 
the air-lock to the shield-chamber. On Nov. 17 the shield was 
all assembled, and riveting the tail-end sections was commenced. 
For heating the rivets in the air-chamber a forge was used, 
with a hood to which was connected at the top a 2-in. pipe with 
a valve which extended through the air-lock bulkhead. By 



SUBMARINE TUNNELING 221 

means of this pipe all the obnoxious gases from the furnace 
were removed from the air-chamber. After the riveting was 
finished, the shield was brought to its right position for line 
and grade, the hydraulic jacks and valves put in place, and the 
necessary connections made. On Nov. 24 word was received 
that the work on the New York side was to be pushed with all 
possible speed, and a force was at once organized of three 
gangs, working in eight-hour shifts. More rings were built on 
the ten rings already anchored in the rock, until the tunnel 
linina was brouofht within the tail-end of the shield. 

The shield was now advanced until it was necessary to 
disturb the bulkhead, the remaining bench ahead of the shield 
being blasted oat as the shield progressed. The most difficult 
part of the work was now reached, for at the point where the 
shield entered the soft, black mud on top there still remained 
about 12 ft. of hard rock in the bottom, as the dip of tliis vein 
was over 40° toward Long Island. Blasting had therefore to 
be continued in the bottom pockets of the shield after the top 
had entered the much-softened material. As soon as the bulk- 
head was passed it was with great difficulty that the bottom 
pockets could be kept clear of the black slush from overhead. 
The material had become so softened along the rock face that 
it was almost impossible to confine it, and several rushes of 
inflowing material occurred, until finally an open connection 
with the river was established, and the tunnel was visited by 
crabs and mussels, together w^ith boulders, old boots and shoes, 
brick, and tinware, direct from the river bottom. Notwith- 
standing these adverse circumstances the work was still pro- 
gressing, although in 45 lbs. of compressed air, which was now 
escaping through the heading, and causing a very violent 
ebullition on the river surface. This upward current of air 
held in check the downward current of water, so that no efforts 
were made to prevent its escape. On Dec. 13 the shield finally 
cleared the rock and was now fully entered into the soft, black 
mud. The main difficulty was now surmounted, the work 



222 TUNNELING 

progressed more rapidly, and the shield soon reached undis- 
turbed material, which was found quite dry and hard. It was 
still the same black mud, with occasional lumps like charcoal^ 
and numerous nodules like pyrites, which glistened like silver 
in the black, peat-like mud. Mattocks were used by the men 
in the working chambers, who would clean out these four com- 
partments to within a foot of the cutting edge. As soon as 
this was done hydraulic pressure was put upon the jacks, some- 
times to the amount of 5,000 lbs. per sq. in., and the shield 
forced ahead 16 or 18 ins., enough for another ring of plates, 
the working chambers again being filled with the displaced 
material. On Dec. 24 the last of the black mud was passed 
through, and lying next to it, at an angle of 40° towards 
Long island, white decomposed feldspar was found, containing 
fragments of decomposed quartz charged with sulphureted 
hydrogen. 

An important departure was now made in the method of 
erecting the cast-iron lining rings by breaking joints with the 
segments. In all the iron-lined tunnels it has been the estab- 
lished custom to erect the rings with continuous horizontal 
joints. For some reason it was thought inadvisable to attempt 
breaking joints with the segments. The writer's experience 
in the Hudson tunnel had shown him the importance of obtain- 
ing, in soft, squeezing ground, a perfectly rigid tunnel-ring. 
In a material exerting hydrostatic pressure the tunnel lining is 
subjected to a resultant strain, tending to flatten the ring, or 
decrease its vertical diameter. Any yielding to this strain 
results both in increasing the deforming pressure and in de- 
creasing the power of the ring to resist the strain. In a lining 
erected with continuous joints the rigidity of the ring is 
dependent upon the bolting in the horizontal joints. At the 
Hudson River tunnel a ring of plates was bolted together 
lying flat on the ground, the plates all brought to a true circle, 
and the two l^-in. bolts in each joint well tightened. Upon 
raising this ring with a derrick, so that it stood erect, the ring 



SUBMARINE TUNNELING 223 

was flattened 3 ins. by its own weight. At the East River 
tunnel a siniihir experiment was made ; two rings of phites 
were bolted together, breaking joints, one ring being revolved 
two holes. These two rings were then raised upright, but no 
flattening could be detected. By means of a turnbuckle a 
measured strain was now brought upon the rings along the 
vertical diameter. At 16,000 lbs. the vertical diameter was 
shortened i-in., the flanges of the plates cracking where the 
turnbuckle was attached. In these two instances there was, of 
course, a great difference in the size of the rings, those in the 
Hudson tunnel being 18 ft. inside diameter, while those in the 
gas tunnel were only 10 ft. 2 ins. inside diameter. 

Aside from the rigidity gained, breaking joints has proved 
much the better in other ways. With continuous joints, two 
things are apt to occur : (1) The joint-face where two rings 
meet may become slightly warped ; that is, all points on this 
face of the ring will no longer lie in the same plane. This 
may be caused by carelessness in allowing dirt to get into the 
joints between the rings. When this once occurs the warping 
increases with every additional ring till true joints can no 
longer be made. (2) The rings may be erected so as to depart 
gradually from a true circular form. This latter case is im- 
possible where the joints are broken, and, in the former in- 
stance, by breaking joints, the error is divided and distributed 
around the ring until it disappears. 

On Jan. 16, 1894, the end of the soft seam was reached with 
the shield, and rock was again entered after having passed 
through 98 ft. of soft ground. This rock resembled slightly 
the rock on BlackwelFs Island. It was in a much shattered 
condition, with many loose heads and small, soft veins. As 
this material required support in the heading and a permanent 
lining, and as, in its present condition, there was no assurance 
that it might not again pass into soft material — shield tunnel- 
ing was still continued. Small machine-drills were set up in 
the four working-chambers of the shield upon arms bolted to 



224 TUNNELING 

the vertical platform, and the rock was drilled and blasted just 
ahead of the shield. The progress of 4 ft. per day was made 
in this material, of which there was about 65 ft. The rock 
then became much more solid, with a roof that was self-sustain- 
ing, and arrangements were made for removing the shield. 
On Feb. 18 the work of removing the shield was begun, and 
two days hiter everything was ready for the regular rock-tunnel 
work in the heading, the shield having been taken apart and 
removed in that time. 

At about the time that shield tunneling was being discon- 
tinued at New York, it was being installed at Long Island. 
An entire duplicate plant had been ordered for this side ; for, 
although it had been originally intended to use one shield for 
both headings, it was later deemed advisable to provide a shield 
for each heading, so that there might be no delay, should soft 
ground be met in both headings at the same time. In passing 
through the soft seam at Ravenswood with the shield, no 
especial difficulties were met. The material proved to be a 
mass of soft-rock fragments, boulders and cinder-like stones im- 
bedded in soft green chlorite. About a month was consumed 
in passing through this seam, removing the shield, and prolong- 
ing the cast-iron lining well into the rock on both sides of 
the vein. With both tunnel headings now in rock, remarkably 
rapid progress was made ; and as progress now had become of 
great importance to the company, a liberal bonus, arranged on 
a sliding scale, was given the foremen for work done over stated 
amounts. Up to the time of the headings meeting, an average 
progress of 69 ft. per week was made, while in rock, on both 
the New York and Long Island sides. The record week of 
the work was the one ending June 27, when at Ravenswood 
95 ft. was driven, while on the New York side, the heading 
was advanced 101 ft., making a total for the week of 196 ft. of 
tunnel driven. Soon after the rock tunneling had been re- 
sumed on the New York side, this heading reached Blackwell's 
Island, and the troubles on this side were over. But at Ravens- 



SUBMARINE TUNNELING 225 

wood, with the heading in white limestone, there was every 
reason to expect further soft seams where the rock should 
change to the granite gneiss of BlackwelFs Island. These- 
expectations were not disappointed ; for after passing through 
350 ft. of the limestone, and when within 200 ft. of Blackwell's- 
Island, a soft seam was met, and air pressure had to be once- 
more used in the heading. As this seam was but 14 ft. in 
width, and presented no especial difficulties, the tunnel was. 
carried through it without using the shield, the cast-iron seg- 
ments being erected under a timber roof. Gneiss was encoun-^ 
tered on the other side of this soft vein, which brought with it 
the assurance that the last of the soft ground had been passed. 
On May 16 serious loss and delay were caused by a fire which 
destroyed the New York works. The fire started in an adjoin- 
ing picnic ground, containing many light frame structures, 
which caused so fierce a conflagration that it was impossible to 
save our works. This caused a delay of three weeks in the 
time of the tunnel's completion. On July 11, 1894, the re- 
maining 15 ft. of rock between the headings was blasted away,, 
thus opening the pioneer tunnel under the East River, two 
years from the time when ground was first broken. Some 
weeks were spent in clearing up and shutting out the water in 
the wet places. A 3-ft. gas main was now laid through to 
New York, and on Oct. 15 gas was delivered into the city^ 
accomplishing the purpose of the tunnel. 

VAN BUREN STREET TUNNEL, CHICAGO. 

The Van Buren Street tunnel in Chicago belongs to that 
class of submarine tunnels wliich has been designated as 
tunnels on the river l)ed, ])y which it is meant that the top of 
the tunnel is flush with, or extends slightly above, the bed of 
the stream. Two methods are available for constructing these 
tunnels : viz., the cofferdam method and the caisson method. 
The cofferdam metliod has been actually employed in several 



226 TUNNELING 

instances ; but the caisson method, although proposed for sev- 
eral projected works, has never actually been employed. The 
Van Buren Street tunnel, built to carry a double-track street 
railway under the Chicago River, was completed in 1894 by 
the cofferdam method. The special features of the tunnel * 
are : (1) the unusually large dimensions of the cross-section of 
30 ft. X 15 ft. 9 ins. ; (2) its construction inside of coffer- 
dams of great length and wdith; (3) the construction under 
some very high buildings calling for great care and very strong 
temporary and permanent supports. 

The special feature of the work for our present purpose 
was the construction of the tunnel across the river. To accom- 
plish this a cofferdam was built out from the west shore of the 
river to its middle, and the tunnel constructed within it like 
the building of any other structure within a cofferdam. Trans- 
verse and longitudinal sections of this cofferdam are shown by 
Fig. 119. As will be seen, it was a simple double- wall coffer- 
dam, with a clear width between the walls of 58 ft., and braced 
transversely as shown. Inside of this a single-wall cofferdam 
of piles was constructed, with a clear width just sufficient to 
allow the construction of the masonry within it. When the 
tunnel end reached the channel end of the cofferdam, a crib-wall 
was built over the end of the completed tunnel, as shown by 
the drawings. This crib wall was intended to form the end 
wall of another cofferdam, which was built out from the east 
shore, and within which the remaining half of the tunnel was 
built as the first half had been. The drawings show the char- 
acter of the tunnel masonry and of the centering upon which 
it was built. 

In this connection it will be interesting to mention briefly 
the most pretentious proposition for tunnel construction by 
means of caissons. Some years ago, Prof. Winkler proposed 
to construct a tunnel under the River Danube to connect the 
various portions of the Vienna, Austria, underground railway. 

* " Eng. News," April 12, 1892. 



SUBMARINE TUNNELING 

'lonq.3'0''C.toC. .^:^ ' ! I , Iji' liU gj-^,' 



227 



Piles 45 




228 TUNNELING 

and to use caissons in the construction. Prof. Winkler pro- 
posed to build caissons from 30 ft. to 45 ft. long, with a width 
depending upon the lateral dimensions adopted for the tunnel 
masonry. The caisson was to be made of metal plates and 
angle iron with riveted connections on all sides except those 
running vertically transverse to the tunnel axis, whose connec- 
tions were to be bolted. The roof of the caisson was to be 
made of T-irons resting upon templates placed on the edge 
of the longitudinal sides of the caisson, and strutted in the 
middle by the crown of an iron arch having its springers upon 
brackets inserted on the vertical angle irons forming the frame 
of the caisson. Between the T-irons of the roof small brick 
vaults were to be built, and a very thick stratum of concrete 
laid on their extrados so as to obtain a level surface. In the 
middle of the roof an opening was to be left ; this was for the 
shaft having the air-locks to allow the passage of men, mate- 
rials, and compressed air. 

Across the river two parallel rows of piles were to be 
driven into the river bed, to fix the place where the caisson was 
to be sunk. Then the first caisson near the shore was to be 
lowered in the ordinary way, and a second caisson was to be 
immediately sunk very close to the first one. When both cais- 
sons had reached the plane of the tunnel floor, the sides which 
were in contact were to be unbolted and removed, and the 
small space between made water-tight by filling them with yarn 
and tar. The chambers of the two caissons were to be opened 
into a single large one communicating above by means of two 
shafts. At the same time that the masonry was being built in 
the two first caissons, from the inverted arch up, a third cais- 
son was to be sunk ; and when by excavation it had reached 
the plane of the projected tunnel floor, the partitions were 
to be removed so that the three caissons were in communica- 
tion, forming a large single caisson. To limit the compressed 
air to the working-place, walls were to be built across the tun- 
nel near the advanced part completely lined. The first wall 



SUIJMARINE TUNNELING 229 

was to be built after four caissons were sunk. Then the outer 
partition of the tirst caisson was to be removed, and the ma- 
sonry of the submarine tunnel connected with the portion of 
the tunnel built on land. In a similar manner all the caissons 
were to be sunk ; and when the last one was placed, and the ma- 
sonry lining constructed, and connected with the portion of 
the tunnel built on the other shore of the river, the partition 
walls were to be battered down, and the submarine tunnel com- 
pletely constructed and open to traffic. 



230 TUNNELING 



CHAPTER XX. 

SUBMARINE TUNNELING (Continued). —THE 
MILWAUKEE WATER-WORKS TUNNEL. 



The new water supply intake tunnel for the city of Mil- 
waukee, Wis., is one of the most difficult examples of tunnel 
construction which American engineering practice has afforded. 
The difficulties were in a large measure unexpected when the 
work was decided upon and put under way. The tunnel began 
and ended in a hard, impervious clay, practically a rock, and 
all the preliminary investigations led to the conclusion that 
the same favorable material would be encountered for its 
entire length. With such material a brick-lined tunnel 7i ft. 
in diameter presented no unusual problems ; but after about 
1,640 ft. had been excavated from the shore end the tunnel 
ran out of the hard clay, and for the next 600 ft. or more 
a variety of water-bearing material was encountered, which 
tried the skill and patience of the engineer to their utmost. 
Other difficulties were indeed met with, but these were of minor 
importance in comparison with that of safely and successfully 
penetrating the water-bearing drift. 

The work of sinking the shore shafts and excavating the 
first 1,600 ft. of tunnel did not prove especially difficult. A 
hard, compact, and rock-like clay, bearing very little moisture, 
was encountered all along, and was blasted and removed in the 
ordinary manner. The only mishap which occurred with this 
portion of the work was the destruction of the contractor's 
boiler plant by fire on Jan. 12, 1891, which allowed the tunnel 
to fill with water, and delayed work about a month. By 
Oct. 21, 1891, 1,640 ft. had been driven, averaging about 6f ft. 



SUBMAKINE TUNNELING 231 

per day, all in the hard clay. No timbering had been necessary, 
and except for the first 100 ft. of the tunnel there was very 
little seepage. On the afternoon of Oct. 21 water was observed 
coming out from one of the drill holes in the heading, but no 
attention was paid to it. Shortly after a blast was fired, and 
was immediately followed by a rush of water from the heading. 
An unsuccessful attempt was made to check the flow, and the 
pumps were started ; but they were unable to keep the water 
down, and after seven hours' hard work the tunnel was aban- 
doned. By the next morning the tunnel and shaft were full of 
water. 

Several attempts were made to empty the tunnel; but the 
limited pumping capacity was not equal to the task, and it was 
finally decided to install larger pumps. The pumping had, how- 
ever, shown that about 1,000 gallons of water a minute was 
coming through the leak. With the increased pumping plant 
the tunnel was finally laid dry Feb. 13, 1892. Upon examina- 
tion the head of the drift was found to be in the same undis- 
turbed condition in which it was left when the water broke in 
three months before. 

A brick bulkhead was built into the end of the brickwork 
of the tunnel, and provided with a timber door for passage, and 
two 10-in. pipes for the outlet of the water. With these open- 
ings closed, the flow was checked sufficiently to allow the pla- 
cing of pumps at the bottom of the shore shaft. Meanwhile the 
pressure of the water against the bulkhead caused dangerous 
leakage, and so after the pumps were in position the 10-in. pipes 
were opened, relieving the pressure and allowing the water its 
normal rate of flow. Trouble with the pumps now arose, and 
after various stoppages and breaks the discharge pipe finally 
fell, disabling the whole plant. It became necessary to close 
the 10-in. pipes in the bulkhead and draw up the pumps. This 
allowed the tunnel to again fill with water. 

After thoroughly overhauling the pumping machinery, the 
contractor again laid the tunnel dry on March 19; and after 



232 



TUNNELING 



the pumps had been permanently placed so as to take care of 
the water, an examination of the work was made. It was found 
that the water was coming from the north, and with the hope 
•of avoiding the difficulties of the old heading, it was decided to 
make a detour of the south. On April 16 work was begun at 
:a point about 90 ft. back fron; the face, and deflecting the Ime 
about 38° toward the south. About 38 ft. from the angle of 
junction a brick bulkhead with two 8-in. openings was built 




Eno.News., 
Fig. 120. — Sketch showing undergroxTiid stream, Milwaukee Water- Works Tunnel. 

into the new bore. The work progressed successfully for about 
75 ft., when water was again encountered ; and upon pushing 
forward the heading, gravel and sand came in such quantities 
i^hat it was found impracticable to continue the work further, 
'On June 1 the bulkhead was permanently closed, and the work 
in this direction was abandoned. 

A further and closer examination was now made of the 
lieading first abandoned. Upon breaking through the rock-like 
clay it was found that the water came from an underground 



SUBMAIIINE TUNNELING 233 

stream flowing from the north through a well defined channel 
in red clay. This channel was about 13 ft. above the grade of 
the tunnel ; and above it in every direction visible was a bed of 
hard, dry, red clay, while immediately in front of the face of the 
work was a bank of coarse gravel. Fig. 120 is a sketch of the 
channel and stream where they entered the work. In this last 
drawing the photograph has been followed exactly, no particu- 
lar being exaggerated in the slightest. The water from this 
stream was clear and pure ; and a chemical analysis showed 
that it was not lake water, but must come from some separate 
source. 

While the engineer did not consider the difficulty of pro- 
ceding along the old line insurmountable, it was decided to ' be 
less difficult on the whole to go back from 150 ft. to 175 ft. and 
deflect the line to the north and upward, so as to pass over the 
underground entrance. Instead of allowing the water to flow 
at its normal rate and take care of it by pumping, the contrac- 
tors desired to reduce the pumping, and to this end they con- 
structed a bulkhead just west of the deflection toward the 
south with a view of shutting off the w^ater. The water, how- 
ever, accumulated with a pressure of some 50 lbs. per sq. in., 
and penetrated the filling around the brick lining of the tunnel, 
preventing the cutting through of the lining for the new line. 
A second bulkhead was then built about 20 ft. west of the 
first, but mth not much l)etter results, for upon closing it the 
water was found to leak through the brickwork for a long 
distance west. Finally on Aug. 2, 1892, the contractors 
lifted their pumps and allowed the tunnel to fill again with 
water. 

No further work was done on the tunnel by the contractors, 
although they continued work on the lake shaft for some 
months. Difficulties had, however, arisen here, which will be 
described further on ; and finally a disagreement arose between 
the contractors and the city over the delay in prosecuting 
the tunnel work and over one or two other questions, which 



234 " TUNNELING 

resulted in the City Council suspending their contract and 
ordering the Board of Public Works to go ahead with the 
work. 

The first step to be taken by the engineer was to purchase 
adequate pumping machinery and empty the tunnel. This was 
effected Jan. 17, 1894 ; and as soon as practicable thereafter the 
two bulkheads were removed and the tunnel cleaned, tram-car 
tracks laid, and everything prepared for work. It was now 
determined to go ahead on the original line of the tunnel if 
possible, and the bulkhead here was removed and work begun. 
Meanwhile, a safety bulkhead had been built to replace the first 
one torn away. This was provided with a door and drain- 
age pipes. Work was begun on the original heading, but had 
proceeded only a little way when the water broke in, driving 
out the workmen. This was removed three or four times, when 
the flow suddenly increased to 3,000 gallons per minute. An 
examination of the lake bottom above the break showed that it 
had settled down, indicating that the new break connected with^ 
the lake bottom, and making further work along the original 
line out of the question. 

The question now arose what it was best to do. It was 
impracticable to use a shield, as the material ahead of the break 
required blasting, and the pressure from above was enormous. 
On account of its expense and difficulty of application the 
freezing process did not seem advisable, and the plenum process 
was likewise out of the question on account of the great 
pressure which would be required at this depth. The detour 
to the south which had been made by the contractor had been 
unsuccessful, and had left the ground in a treacherous condi- 
tion. To depress the tunnel was not advisable, for it was not 
by any means certain that the bed of gravel could be avoided 
in that way ; and, moreover, it would be necessary to ascend 
again further on, and thus leave a trap which would effectually 
cut off escape to those at work on the face if water again broke 
into the tunnel. 



SUBMARINE TUNNELING 235 

It was finally decided that the old plan of deflecting the 
line toward the north and upward so as to pass over the under- 
ground stream should be tried. A hole was therefore cut 
through the tunnel lining 1,433 ft. from the shore, and work 
was begun on a detour of 20° toward the north and an upward 
grade of 10 %. Fair progress was made on this new line, 
gradually ascending into solid rock, until May 10, when the 
test borings, which were constantly made in every direction 
from the face, showed that sand was being approached. A 
brick bulkhead was therefore built into the masonry as a safe- 
guard, should it happen that water was encountered in large 
quantities. As the borings seemed to indicate that the top 
surface of the rock underlying the sand was nearly level, the 
lower half of the tunnel was first excavated, leaving about 18 
ins. of the rock to serve as a roof (Sketch a. Fig. 121), and the 
brick invert was built for a distance of 52 ft. The rock roof 
was then carefully broken through for short distances at a time, 
and short sheeting driven ahead into the sand, which proved to 
be a very fine -quicksand flowing through the smallest openings. 
Extreme care had to be taken in this work, but little by little 
the brickwork was pushed ahead until at a distance of 90 ft. 
from the point where the sand was first met, and 208 ft. from 
the old tunnel, the sand stopped and the heading entered a 
hard clay. 

All this work had been done on an ascending grade, and the 
ascent was continued about 40 ft. farther in the clay. By this 
time a suflicient elevation was gained to pass over the under- 
ground stream, and the tunnel line was changed to head toward 
the lake shaft, and the grade reduced to a level. The under- 
ground stream was passed without trouble and the tunnel 
continued for a distance of 54 ft. without difficulty. On July 
10 the clay in the heading suddenly softened, and before the 
miners could secure it by bracing, the water rushed in, followed 
by gravel, filling up solidly some 34 ft. of the tunnel before it 
was stopped Ijy a timber bulkhead hastily built. 



236 



TUNNELING 



Upon examining the lake bottom a cavity over 60 ft. deep and 
10 ft. in diameter was found directly over the end of the tunnel, 
which had been caused by the gravel breaking into the tunnel. 
Having now reached an elevation where it was possible to use 
compressed air, it was determined to put in double air-locks 
and use the plenum process. The locks were built, and some 




// / Longitudinal Section Showing Method of 

Construction in Rock Covered with Quicksand 

Sketch "a". 



^^^i.^' 



Section A-B-C-D. 
Sketch "c". 





"O ^Boulders 



Cross Section Showing Manner of 
Constructing Lining around Boulder. 

Sketch, "b*. Sketch "d." 

Pig. 121.— Sketch Showing Methods of Lining, Milwaukee Water- Works Tunnel. 

670 cu. yds. of clay were dumped into the hole in the lake 
bottom. On Aug. 4 the air-locks were tried with 26 lbs. air 
pressure; but, upon a temporary release of the pressure, the 
water passed around the locks and back of the tunnel lining 
for some distance, and even forced through the lining, carrying 
considerable clay and fine sand with it. TTpon sounding the 



SUBMARINE TUNNELING 237 

lake bottom it was found that the cavity had again increased 
to a deptli of 60 ft., whereupon an additional 600 cu. yds. of 
clay were dumped into it. 

On account of the water leaking through the brickwork, the 
only dry place to cut through the brickwork and build in an 
air-lock was just ahead of the brick bulkhead. This lock was 
completed Aug. 27, and to avoid encountering the danger of 
the direct connection with the lake at the end of the drift, it 
Avas decided to make another detour to the north. On Aug. 28, 
therefore, the brick on the north side of the tunnel 12 ft. back 
from the end of the brickAvork was cut through under 25 lbs. 
air pressure, and work proceeded in good, hard clay. The 
original air-lock was cut out and a new lock built into this 
clay about 34 ft. from the last detour, to be used in case of 
further difficulties. After buildino- the tunnel for about 80 ft. 
from the detour, the soundings again indicated the approach to 
gravel and water, and on Oct. 14 the water broke through from 
the bottom in such volume and with such force that the men 
ran out, closing every air-lock and the valves of every drain in 
their haste to escape, until the brick bulkhead was reached. 
It was Avith great difficulty that the portion of the tunnel up to 
the last air-lock was recovered and cleaned out. 

It was now recognized that a pressure of from 38 to 40 lbs. 
of air would be needed to hold this water, and accordingly an- 
other compressor was added to the plant. With a pressure of 
36 lbs. the water was driven out and the work again started. 
At this time also an additional 350 cu. yds. of clay were dumped 
into the hole in the lake bottom. Altogether, 1,620 cu. yds. 
of clay had been put into this hole. 

Loose gravel and boulders, some of innnense size, were now 
encountered, and the work became exceedingly difficult on 
account of the great escape of air. The interstices between the 
gravel and boulders were not filled with silt or sand, but con- 
tained water. Moreover, this material extended upward to the 
lake bottom, as was shown by the escape of air at the surface of 



288 TUNNELIls^G 

the lake. For an area of several hundred square feet the surface 
of the water resembled a pot of boiling water. At times the 
air would escape very rapidly, and again only a few bubbles 
would show. 

It need hardly be said that the work in this gravel was very 
slow. It was impossible to blast or to tear out the large boulders 
whole, as so much surface would be exposed that an inrush of 
water would take place despite the air pressure. The method 
of procedure was to excavate a heading and build the brick roof 
arch first, and then to take out the bench and build the in- 
vert. Fig. 121 gives a number of sketches showing how the 
work was done. A short piece of heading was taken out, the 
top and face of the bench being meanwhile plastered with clay 
(Sketches h and <?, Fig. 121) to reduce the escape of air, and 
then the roof arch was built and supported on side sills resting 
on the bench. Bit by bit the roof arch was pushed forward 
until some little distance had been completed, then the heading 
was plastered with clay and the bench taken out little by little 
and the invert built. All the gravel except the small area 
upon which work was actually in progress was kept thoroughly 
plastered with clay ; and as the air escaped through the com- 
pleted brick work very rapidly, water was allowed to cover a 
portion of the invert (see Sketch <?, Fig. 121), so as to reduce 
the area of escape. 

When a large boulder was reached, which lay partly within 
and partly without the tunnel section, the lining was built out 
and around it, as shown in Sketch cZ, Fig. 121. The boulder 
was then broken and taken out. All through this gravel bed 
the cross-section of the lining is made irregular by the con- 
struction of these pockets in the lining to get around boulders. 
Sometimes they were on one side and sometimes on the other, 
or on both, or at the top or bottom. In fact, there was no 
regularity. Despite the hazard and danger of this work, con- 
tinual progress was made, though sometimes it was only 4 ft. 
of completed tunnel per week, working night and day ; and, if 



SUBMARINE TUNNELING 239 

some cases of caisson disease be excepted, the only mishap oc- 
curring was a fire which got into the timber packing behind 
the lining and caused some trouble. From the gravel the tunnel 
ran into clay and quicksand, and then into hard, dry clay 
similar to that encountered near the shore. Some difficulty 
was had with the quicksand, but it was successfully overcome ; 
and when the hard clay Avas struck, the trouble, as far as the 
work from the shore shaft was concerned, was virtually over. 

Meanwhile, a different set of afflictions had come upon the 
engineei" and contractors in sinking the lake shaft and driving 
the heading toward shore. This shaft was intended to be 
built by sinking a cast-iron cylinder 10 ft. in diameter, made 
up of sections bolted together. Work was begun July 5, 1892, 
and the sinking was accomplished first by weighting the cylinder, 
and afterwards by pumping out the sand and water within it 
until the pressure from the outside broke through under the 
cutting edge and forced the sand into the cylinder, allowing it 
to sink a little. From 10 to 30 cu. yds. of sand were carried 
into the cylinder each time, and finally it was feared that if 
the process continued, the crib, which had been previously 
erected, would be undermined. On Sept. 6, therefore, the 
contractors were ordered to discontinue this method of work. 
No change was made, however, until Oct. 1, when the cylinder 
had reached a depth of 68 ft., and by this time there was quite 
a large cavity underneath the crib. This was refilled, and the 
cylinder pumped out, and excavation begun inside of it. On 
Oct. 11 a 2J-ft. deep ring of brick work was laid underneath 
the cutting edge ; ])ut in trying to put in another ring beneath 
the first, two days later, the sand and water broke through the 
bottom, driving the men out, and filling the cylinder to a depth 
of 16 ft. with sand. The pumps were started, but the water 
could not be lowered to a greater depth than 60 ft. 

At the request of the contractors, the city engineer had a 
boring made at the center of the shaft to determine the 
character of the material to be further penetrated. This 



240 tu:nneling 

boring showed that sand mixed with loam and gravel would be 
found for a depth of 26 ft., then would come 15 ft. of red clay, 
and finally a layer of hard clay like that penetrated by the 
shore end of the tunnel. About the middle of December the 
contractors made another attempt to pump the shaft, but find- 
ing that the water came in at the rate of 25 gallons a minute, 
abandoned the attempt. In the latter part of February prepa- 
rations were made to put an air-lock in the shaft and use 
compressed air. Hardly had the work been begun by this 
system, when, on April 20, 1893, a terrific easterly storm, swept 
the top of the crib bare of the buildings and machinery, and 
drowned all but one of the 15 men at work there. 

This disaster delayed the work for some time, but in June 
the contractors erected a new building and new machinery, and 
resumed work. Very little progress was made ; and the air es- 
caped so rapidly that it loosened the sand surrounding the 
shaft and reduced the friction to such an extent that on July 
28 the entire cylinder lifted bodil}^ about 6 ft., and sand rushed 
in, filling the lower part of the cylinder to within 45 ft. of the 
lake surface. No further work was done by the contractors, 
although they submitted a proposition to sink a steel cylinder 
inside the cast-iron cylinder and extending from 5 ft. above 
datum to 100 ft. below datum for $300 per ft. This proposi- 
tion was refused by the city; and since Avork on the tunnel 
proper has been abandoned by the contractors some time before, 
as had already been described, the city suspended their contract 
on Oct. 19. 

On Oct. 30 a contract was made with Mr. Thos. Murphy, 
of Milwaukee, Wis., to sink a steel cylinder inside the old iron 
cylinder. The water was first pumped out of the old cylinder, 
and a timber bulkhead built at the bottom. On this the steel 
cylinder was built, and then the bulkhead was removed. Air 
pressure was put on, and the excavation proceeded successfully 
until the bottom layer of clay was met with, when all chances 
for trouble ceased. 



SUBMAK1>'E TUNNELING 241 

The cylinder, as it was completed, penetrated 9 ft. into the 
hard clay, and was underpinned with brickwork for a depth of 
29 ft. or more, to a point -i ft. below the grade line of the 
tunnel. At the lower end, the section of the shaft was changed 
from a circle to a square. Later the steel cylinder was lined 
with brick. 

On ^larch 28, 1894, an agreement was made with Mr. 
Thos. iNIurphy to construct the tunnel fj'om the lake shaft 
toward the shore. Except that considerable water was en- 
countered, which, owing to inadequate pumping machinery, 
filled the tunnel and shaft at two different times, and had to 
be removed, no very great difficulty was had with this part of 
the work. 

On July 28, 1895. the headings from the lake and shore 
shafts met. Meanwhile the cast-iron pipe intake, the intake 
crib, etc., had been completed, and practically all that remained 
to be done was to clean the tunnel and lift the pumping 
machinery at the shore shaft. During the cleaning, the air 
pressure had been kept up on account of the leakage through 
the brick lining, and, indeed, the pressure was kept up until 
the last possible moment, and everything made ready for 
removing the air locks, bulkheads, pumps, etc., in the least 
possible time. The pumps were the last to come out. 



542 TUNNELING 



CHAPTER XXL 

SUBMARINE TUNNELING (Continued). — THE 
SHIELD SYSTEM. 



Historical Introduction. — The invention of the shield system 
Df tunneling through soft ground is generally accredited to Sir 
Isambard Brunei, a Frenchman born in 1769, who emigrated to 
the United States in 1793, where he remained six years, and 
then went to England, in which country his epoch-making in- 
Tention in tunneling was developed and successfully employed in 
building the first Thames tunnel, and where he died in 1849, a 
few years after the completion of this great work. Sir Isambard 
is said to have obtained the idea of employing a shield to tunnel 
soft ground from observing the work of ship-worms. He no- 
ticed that this little animal had a head provided with a boring 
apparatus with which it dug its way into the wood, and that its 
body threw off a secretion which lined the hole behind it and 
Tendered it impervious to water. To»duplicate this operation 
by mechanical means on a large enough scale to make it ap- 
plicable to the construction of tunnels was the plan which 
occurred to the engineer ; and how closely he followed his ani- 
mate model may be seen by examining the drawings of his 
first shield, for which he secured a patent in 1818. Briefly 
(described, this device consisted of an iron cylinder having at 
its front end an auger-like cutter, whose revolution was in- 
tended to shove away the material ahead and thus advance the 
■cylinder. As the cylinder advanced the perimeter of the hole 
.behind was to be lined with a spiral sheet-iron plating, which 
was to be strengthened with an interior lining of masonry. It 
Tivill be seen that the mechanical resemblance of this device to 



SUBMARINE TUNNKLINC; 243 

the ship-worm, on which it is alleged to have been modeled, was 
remarkably close. 

In the same patent in which Sir Isambard secured protection 
for his mechanical ship-worm lie claimed equal rights of inven- 
tion for another shield, which is of far greater importance in 
being the prototype of the shield actually en.ployed by him in 
constructing the first Thames tunnel. This alternative inven- 
tion, if it may be so termed, consisted of a group of separate 
cells which could be advanced one or more at a time or all 
together. The sides of these cells were to be provided with 
friction rollers to enable them to slide easily upon each other; 
and it was also specified that the preferable motive power for 
advancing the cells was hydraulic jacks. To summarize briefly, 
therefore, the two inventions of Brunei comprehended the pro- 
tecting cylinder or shield, the closure of the face of the exca- 
vation, the cellular division, the hydraulic-jack propelling power, 
and cylindrical iron lining, which are the essential characteris- 
tics of the modern shield system of tunneling. The next step 
required was the actual proof of the practicability of Brunei's 
inventions, and this soon came. 

Those who have read the history of the first Thames tunnel 
will recall the early unsuccessful attempts at construction which 
had discouraged English engineers. Five years after Brunei's 
patent was secured a company was formed to undertake the 
task again, the plan being to use the shield system, under the 
jjersonal direction of its inventor as chief engineer. For this 
work Brunei selected the cellular shield mentioned as an alter- 
native construction in his original patent. He also chose to 
make this shield rectangular in form. This choice is commonly 
accounted for by the fact that the strata to be penetrated by the 
tunnel were practically liorizontal, and that it was assumed by 
the engineer tliat a rectangular shield would for some reason 
best resist the pressures which would be dev(!loj)C(l. Whatever 
the reason may have been for the choice, the fact remains that 
a rectangular shield was adopted. The tunnel as designed con- 



244 TUNNELING 

sisted of two parallel horseshoe tunnels, 1 3 ft. 9 ins. wide and 
16 ft. 4 ins. high and 1200 ft. long, separated from each other 
by a wall 4 ft. thick, pierced by 64 arched openings of 4 ft. 
span, the whole being surrounded with massive brickwork built 
to a rectangular section measuring over all 38 ft. wide and 
22 ft. high. 

The first shield designed by Brunei for the work proved in- 
adequate to resist the pressures, and it was replaced by another 
somewhat larger shield of substantially tlie same design, but of 
improved construction. This last shield was 22 ft. 3 ins. high 
and 37 ft. 6 ins. wide. It was divided vertically into twelve 
separate cast-iron frames placed close side by side, and each 
frame was divided horizontally into three cells "capable of sepa- 
rate movement, but connected by a peculiar articulated con- 
struction, which is indicated in a general way by Fig. 122. To 
close or cover the face of the excavation, ^^oling-boards held in 
place by numerous small screw-jacks were employed. Each 
cell or each frame could be advanced independently of the 
others, the power for this operation being obtained by means 
of screw-jacks abutting against the completed masonry lining. 
Briefly described, the mode of procedure was to remove the 
poling-boards in front of the top cell of one frame, and excavate 
the material ahead for about 6 ins. This being done, the top 
cell was advanced 6 ins. by means of the screw-jacks, and the 
poling-boards were replaced. The middle cell of the frame was 
then advanced 6 ins. by repeating the same process, and finally 
the operation was dupUcated for the bottom cell. With the 
advance of the bottom cell one frame had been pushed ahead 
6 ins., and by a succession of such operations the other eleven 
frames were advanced a distance of 6 ins., one after the other, 
until the whole shield occupied a position 6 ins. in advance of 
that at which work was begun. The next step was to fill the 
6-in. space behind the shield with a ring of brickwork. 

The illustration, Fig. 122, is the section parallel to the ver- 
tical plane of the tunnel through the center of one of the 



SUBMARINE TUNNELING 



245 



frames, and it shows quite clearly the complicated details of 
tlie shield construction. Two features which are to be particu- 
larly noted are the suspended staging and centering for con- 



^^^^^^^fe^^^^^l^^^ ^^^l^^ ^^^i^ ^^p 







Fio. 122. — Loiifritndinal SM-tion ofBnniel's Sliicld. First Tli;iines Tunnel. 

Stnicting the roof aich, and tlie top plate of the shield extending 
back and overlappincr the roof masonry so as to close completely 
the roof of the excavation and prevent it fallincr. Notwithstand- 
ing its complicated construction and unwieldy weight of 120 



246 



TUNNELING 



tons, this shield worked successfully, and during several months 
the construction proceeded at the rate of 2 ft. every 24 hours. 
There were two irruptions of water and mud from the river 
during the work, but the apertures were effectually stopped by 
heaving bags of clay into the holes in the river bed, and cover- 
ing them over with tarpauling, with a layer of gravel over all. 
The tunnel was completed in 1848, at a cost of about f 56 00 
per lineal yard, and 20 years from the time work was first 
commenced, including all delays. 

The next tunnel to be built by the shield system was the 
tunnel under London Tower constructed by Barlow and 
Greathead and begun in 1869. In 1863 Mr. Peter W. Barlow 

secured a patent in England 
for a system of tunnel con- 
struction comprising the use of 
a circular shield and a cylindri- 
cal cast-iron lining. The shield, 
as shown by Fig. 123, was 
simply an iron or steel plate 
cylinder. The cylinder plates 
were thinned down in front to 
form a cutting edge, and they 
extended far enough back at the rear to enable the advance 
ring of the cast-iron lining to be set up within the cylinder. In 
simplicity of form this shield was much superior to Brunei's ; 
but it seems very doubtful, since it had no diametrical bracing 
of any sort, whether it would ever have withstood the com- 
bined pressure of the screw-jacks and of the surrounding earth 
in actual operation without serious distortion, and, probably, 
total collapse. It should also be noted that Barlow's shield 
made no provision for protecting the face of the excavation, 
although the inventor did state that if the soil made it neces- 
sary such a protection could be used. The patent provided 
for the injection of liquid cement behind the cast-iron lining 
to fill the annular space left by the advancing tail-plates of the 




Fig. 123.— First Shield Invented by Barlow. 



SUHMAKINE TUNNELING 



247 



shield. Although Barlow made vigorous efforts to get his 
shield used, it was not until 1868 that an opportunity pre- 
sented itself. In the meantime the inventor had been studying 
how to improve his original device, and in 1868 he secured addi- 
tional patents covermg these improvements. Briefly described^ 
they consisted in partly closing the shield with a diaphragm, 
as shown by Fig. 124. The uninclosed portion of the shield is 
here shown at the center, but the patent specified that it might 
also be located below the center in the bottom part of the 
shield. The idea of the construction was that in case of an 
irruption of water the upper portion of the shield could be 
kept open by air ..pressure, and work prosecuted in this open 
space until the shield ^ 

had been driven ahead ^^ 
sufficiently to close 
the aperture, when 
the normal condition 
of affairs would be 
resumed. This was 
obviously an improve- 
ment of real merit. 
The partial diaphragm 
also served to stiffen the shield somewhat against collapse, but 
the thin plate cutting-edges and most of the other structural 
weaknesses were left unaltered. To summarize briefly the- 
improvements due to Balow's work, we have : the construction 
of the shield in a single piece ; the use of compressed air and 
a partial diaphragm for keeping the upper part of the shield 
open in case of irruptions of water; and the injection of liquid 
cement to fill the voids behind the lining. 

Turning now to the London Tower tunnel work, it may 
first be noted that Barlow found some difficulty in finding a 
contractor who was willing to undertake the jol), so little 
confidence had engineers generally in his shield system. One 
man, however, Mi'. J. II. Oreathead, f)erceived that Barlow's 





Longitudinal Section. Cross Section . 

Fig. 124. — Second Shield Invented by Barlow. 



'248 TUNNELING 

device presented merit, although its design and construction 
were defective, and he finally undertook the work and carried 
it to a brilliant success. The tunnel was 1,350 ft. long and 
7 ft. in diameter, and penetrated compact clay. Work was 
begun on the first shore shaft on Feb. 12, 1869, and the tunnel 
was completed the following Christmas, or in something short 
of eleven months, at a cost of X 14,500. 

The shield used was Barlow's idea put into practical shape 
by Greathead. It consisted of an iron cylinder, or, more 
properly, a frustum of a cone whose circumferential sides 
were very slightly inclined to the axis, the idea being that 
the friction would be less if the front end of the shield were 
slightly larger than the rear end. The shell of the cone was 
made of J in. plates. The thinned plate cutting-edge of 
Barlow's shield was replaced by Greathead with a circular 
ring of cast iron. Greathead also altered the construction of 
the diaphragm by arranging the angle stiffeners so that they 
ran horizontally and vertically, and by fastening the diaphragm 
plates to an interior cast-iron ring connected to the shell plates. 
This was a decided structural improvement, but it was accom- 
panied with another modification which was quite as decided 
a, retrogression from Barlow's design. Greathead made the 
diaphragm opening rectangular and to extend very nearly from 
the top to the bottom of the shield, thus abandoning the 
element of safety provided by Barlow in case of an irruption 
of water. Fortunately the material penetrated by the shield 
for the Tower tunnel was so compact that no trouble was had 
from water ; but the dangerous character of the construction 
was some years afterwards disastrously proven in driving the 
Yarra River tunnel at Melbourne, Australia. To drive his 
shield Greathead employed six 2^ in. screw-jacks capable of 
developing a total force of 60 tons. The tails of the jack bore 
against the completed lining, which consisted of cast-iron rings 
18 ins. wide and ^ in. thick, each ring being made up of a 
crown piece and three segments. The different segments 



SUBMARINE TUNNELING 



249 



and rings were provided A\ith double (exterior and interior) 
flanges, by means of which they were bolted together. The 
soil behind the lining was filled with liquid cement injected 
throuo-h small holes by 

n 




Fig. 125. — Shield Suggested by Greathead for the 
Proposed North and South Woolwich Subway. 



means of a hand pump. 
The remarkable suc- 
cess of the London Tower 
tunnel encourao-ed 

o 

Barlow to form in 1871 a 
company to tunnel the 
Thames between South- 
wark and the City, and 
Greathead, in 1876, to project a tunnel under the same water- 
way known as the North and South Woolwich Subway. Bar- 
low's concession was abrogated by Parliament in 1873, without 

any work having been 
done. Greathead pro- 
gressed far enough witli 
his enterprise to construct 
a shield and a large 
amount of the iron lining 
when the contractors 
abandoned the work. 
From the brief descrip- 
tion of his shield given 
by Greathead to the Lon- 
don Society of Civil En- 
gineers, it contained sev- 
eral important differences 
from the shield built by 
him for tlie London 
Tower tunnel, as is shown 
by Fig. 125. The changes 
which deserve particular notice are the great extension of the 
shield behind the diaphragm, the curved form of the diaphragm. 




Fig. 



12G— Beafh's Shield Tsed on 
Pneumatic Railway Tunnel. 



Broadway 



250 



TUNNELING 



and the use of hydraulic jacks. Greathead had also designed 
for this work a special crane to be used in erecting the cast-iron 
segments of the lining. 

While these works had been progressing in England, Mr. 
Beach, an American, received a patent in the United States for 
a tunnel shield of the construction shown by Fig. 126, which 
was first tried practically in constructing a short length of 
tunnel under Broadway for the nearly forgotten Broadway 




Fig. 127. — Shield for City and South London Railway. 

Pneumatic Underground Ry. This shield, as is indicated by 
the illustration, consisted of a cylinder of wood with an iron- 
cutting-edge and an iron tail-ring. Extending transversely 
across the shield at the front end were a number of horizontal 
iron plates or shelves with cutting-edges, as shown clearly by 
the drawing. The shield was moved ahead by means of a 
number of hydraulic jacks supplied with power by a hand 
pump attached to the shield. By means of suitable valves all 
or any lesser number of these jacks could be operated, and by 



SUBMARINE TU^'NELIJSrG 



251 



>ii 









3? 










SECTION SHOWING HALF OF WALL F. f -.^^-.SEOTON SHOWING HALF OF WALL E. i^«i 

r ml 




252 



TUNNELING 



thus regulating the action of the motive power the direction of 
the shield could be altered at will. Work was abandoned on 
the Broadway tunnel in 1870. In 1871-2 Beach's shield was 
used in building a short circular tunnel 8 ft. in diameter in 
Cincinnati, and a little later it was introduced into the Cleve- 
land water-works tunnel 8 ft. in diameter. In this latter work, 
which was through a very treacherous soil, the shield gave a 
great deal of trouble, and was finally so flattened by the 




Longitudinal Section, Cross Section. 

Fig. 129. — Shield for Blackwall Tunnel. 

pressures that it was abandoned. The obviously defective fea- 
tures of this shield were its want of vertical bracing and the 
lack of any means of closing the front in soft soil. 

With the foregoing brief review of the early development of 
the shield system of tunneling, v^e have arrived at a point where 
the methods of modern practice can be studied intelligently. 
In the pages which follow we shall first illustrate fully the 
construction of a number of shields of typical and special 
construction, and follow these illustrations with a general dis- 
cussion of present practice in the various details of shield 
construction. 



SUBMARINE TUNNELING 



353 




Transverse Section. 




Longitudinal Section^ 

Fig. 130. — PZlliptical Shield for Clichy Sewer Tunnel, Paris. 

Mr. Raynald Legouez, in his excellent book upon the shield 
system of tnnnelinpr, considers that tunnel shields may be di- 
vided into three classes structurally, according to the character 



254 



TUNNELING 



of the material which they are designed to penetrate. In the 
first class he places shields designed to work in a stiff and com- 
paratively stable soil, like the well-known London clay ; in the 
second class are placed those constructed to work in soft clays 
r nd silts ; and in the third class those intended for soils of an 




^^^^^^ 



Longitudinal Section. 




Cross Section. 
Fm. 131. — Semi Elliptical Shield for Clichy Sewer Tunnel. 

unstable granular nature. This classification will, in a general 
way, be kept by the writer. As a representative shield of 
the first class, the one designed for the City and South London 
Ry. is illustrated in Fig. 127. The shields for the London 
Tower tunnel, the Waterloo & City Ry., tlie Glasgow District 
Subway, the Siphons of Clichy and Concorde in Paris, and the 



SUBMAKINK TUNNELING 



:do 



Glasgow Port tunnel, are of the same general design and con- 
struction. To represent sliields of the second class, the St. 
Clair River and Blackwall shields are shown in Figs. 128 and 
129. Tlie shields for the ^lersey River, the Hudson River, 
and the East River tunnels also belong to this class. To 
represent shields of the third class, the elliptical and semi- 




Details of Costing 
■Supporting Ends of Jacks. 



•< IT X 

Details of Casting under Ends Lonqitudlnal Section C-D. 

of Ciirders. ^ 

Fin. 132. — Eoof Sliiekl for Boston Subway. 



elliptical shields of the ( 'lichy tunnel work in Paris are sliown 
by Figs. 130 and 181. The semi-circular shield of tlie Boston 
Subway is illustrated by Fig. 132. 



SHIELD CONSTRUCTION. 

General Form. — Tunnel shields are usually cylindrical or 
semi-cylindriccil in cross-section. The cylinder maybe circular, 
elliptical, or oval in section. Far the greater nund)er of sliields 
used in the past have been circular cylinders; but in one i)ait 
of the sewer tunnel of Clichy, in Paris, an elliptical shield 



256 TUNNELING 

with its major axis horizontal, was used, and the German en- 
gineer, Herr Mackensen, has designed an oval shield, with its 
major axis vertical. A senii-elliptical shield was employed on 
the Clichy tunnel, and semi-circular shields were used on the 
Baltimore Belt Line tunnel and the Boston Subway in Amer- 
ica. Generally, also, tunnel shields are right cylinders ; that is, 
the front and rear edges are in vertical planes perpendicular to 
the axis of the cylinder. Occasionally, however, they are 
oblique cylinders ; that is, the front or rear edges, or both, are 
in planes oblique to the axis of the cylinder. One of these 
visor-shaped shields was employed on the Clichy tunnel. 

The Shell. — It is absolutely necessary that the exterior sur- 
face of . the shell should be smooth, and for this reason the 
exterior rivet heads must be countersunk. It is generally 
admitted, also, that the shell should be perfectly cylindrical, 
and not conical. The conical form has some advantage in 
reducing the frictional resistance to the advance of the shield ; 
but this is generally considered to be more than counterbalanced 
by the danger of subsidence of the earth, caused by the exces- 
sive void which it leaves behind the iron tunnel lining. For 
the same reason the shell plate, which overlaps the forward ring 
of the lining, should be as thin as practicable, but its thickness 
should not be reduced so that it will deflect under the earth 
pressures from above. Generally the shell is made of at least 
two thicknesses of plating, the plates being arranged so as to 
break joints, and, thus, to avoid the use of cover joints, to inter- 
rupt the smooth surface which is so essential, particularly on 
the exterior. The thickness of the shell required will vary 
with the diameter of the shield, and the character and strength 
of the diametrical bracing. Mr. Raynald Legouez suggests as 
a rule for determining the thickness of the shell, that, to a 
minimum thickness of 2 mm., should be added 1 mm. for every 
meter of diameter over 4 meters. Referring to the illustrations, 
Figs. 128 to 132 inclusive, it will be noted that the St. Clair 
tunnel shield, 21^ ft. in diameter, had a shell of 1-in. steel 



SUBMARl^y'E TUNNELING 257 

plates with cover-plate joints and interior angle stiffeners ; the 
shell of the East River tunnel shield, 11 ft. in diameter, was- 
made up of one ^n. and one |-in. plate; the Blackwall tunnel 
shield, 21 ft. 9 ins. in diameter, had a shell consisting of 
four thicknesses of |-in. plates ; and the Clichy tunnel shield,, 
with a diameter of 2.06 meters, had a shell 2 millimeters thick. 
Front-End Construction. — By the front end is meant that 
portion of the shield between the cutting-edge and the vertical 
diaphragm. The length of this portion of the shield was 
formerly made quite small, and where the material penetrated 
is very soft, a short front-end construction yet has many advo- 
cates ; but the general tendency now is to extend the cutting- 
edge far enough ahead of the diaphragm to form a fair-sized 
working chamber. Excavation is far more easy and rapid when 
the face can be attacked directly from in front of the diaphragm 
than where the work has to be done form behind through the 
apertures in the diaphragm. So long as the roof of the excava- 
tion is supported from falling, experience has shown that it is 
easily possible to extend the excavation safely some distance 
ahead of the diaphragm. In reasonably stable material, like 
compact clay, the front face will usually stand alone for the 
short time necessary to excavate the section and advance the 
shield one stage. In softer material the face can usually be 
sustained for the same short period by means of compressed air; 
or the face of the excavation, instead of being made vertical, can 
be allowed to assume its natural slope. In the latter case a 
visor-shaped front-end construction, such as was used on some 
portions of the Clichy tunnel, is particularly advantageous. The 
following figures show the lengths of the front ends of a number 
of representative tunnel shields. 

City and South London . 1 ft. Mersey River 3 ft. 

St. Clair Kiver .... 11.25 " East River 3S '^ 

Hudson "River .... 5ij '* Blackwall 6.5 '^ 

Two general t}7)es of construction are employed for the 
cutting-edge. The first type consists of a cast-iron or cast-steel 



258 



TUNNELING 



ring, beveled to form a chisel-like cutting-edge, and bolted to 
.the ends of the forward shell plates. This construction was 
.first employed in the shield for the London Tower tunnel, and 
has since been used on the City and South London, Waterloo 
.and City, and the Clichy tunnels. The second construction 
^consists in bracing the forward shell plates by means of right 
■> triangular brackets, whose perpendicular sides are riveted 
■respectively to the shell plates and the diaphragm, and whose 
■inclined sides slant backward and downward from the front 
■edge, and carry a conical ring of plating. The shields for the 
■St. Clair River, East River, and Blackwall tunnels show forms 
-of this typ3 of cutting-edge construction. A modification of 
the second type of construction, which consists in omitting the 
conical plating, was employed on some of the shields for the 
Clichy tunnel. This modification is generally considered to be 
allowable only in materials which have little stability, and which 
crumble down before the advance of the cutting-edge. Where 
the material is of a sticky or compact nature, into which the 
shield in advancing must actually cut, the beveled plating is 
necessary to insure a clean cutting action without wedging or 
jamming of the material. 

Cellular Division. — It is necessary in shields of large diam- 
eter to brace the shell horizontally and vertically against 
distortion. This bracing also serves to form stagings for the 
workmen, and to divide the shield into cells. The following 
table shows the arrangement of the vertical and transverse 
bracing in several representative tunnel shields. 



Name of Tu>-nei.. 


Diameter. 


Hori- 
zontal. 


Plates, 

Dl.ST. 

Apart. 


Vert. 
Braces. 


Hudson River 

Clichy 

St. Clair River 

Waterloo (Station) .... 

Blackwall 

East River 


Ft. 

19 

19.4 

21 

24 

27 

11 


In. 
11 


6 

10^ 
8 
!- 


No. 
2 
2 
2 

2 

2 
None 


Ft. 

6.54 

6.54 

6.98 

7.12 

6.0 


No. 

2 
None 

3 
None 

3 

1 



SUBMAIUXE TUNNELING 259 

Referring first to the horizontal divisions, it may be noted 
that tiiey serve different purposes in different instances. In the 
Clichy tunnel shield the horizontal divisions formed simply 
working platforms ; in the Waterloo tunnel shield they were 
designed to abut closely against the working face by means of 
special jacks, and so to divide it into three separate divisions; in 
the St. Clair tunnel they served as working platforms, and also 
had cutting-edges for penetrating the material ahead ; and in 
the Blackwall tunnel shield they served as working platforms, 
and had cutting-edges as in the St. Clair tunnel shield, and in 
addition the middle division was so devised that the two lower 
chambers of the shield could be kept under a higher pressure of 
air than the two upper chambers. Passing now to the veitical 
divisions, they serve to brace the shell of the shield against ver- 
tical pressures, and also to divide the horizontal chambers into 
cells ; but unlike the horizontal plates they are not provided 
with cutting-edges. The St. Clair, Hudson River, and Black- 
wall tunnel shields illustrate the use of the vertical bracing for 
the double purpose of vertical bracing and of dividing the hori- 
zontal chambers into cells. The Waterloo tunnel shield is 
an example of vertical bracing employed solely as bracing. 
The vertical division of the East River tunnel shield was 
employed in order to allow the shield to be dissembled in 
quadrants. 

The Diaphragm. — The purpose of the shield diaphragm is to 
close the rear end of the shield and the tunnel behind from an 
inrush of water and earth from the face of the excavation. It 
also serves the secondary purpose of stiffening tlie shell diamet-' 
rically. Structurally the diaphragm separates the front-end con- 
struction previously described from the rear-end construction, 
which will be described farther on ; and it is usually composed 
of iron or steel plating reinforced by beams or giiders, and 
pierced with one or several 0[)enings by wliich access is had 
to the working face. In stable mateiial, wliere caving or an 
inrush of water and earth is not likely, the diaphragm is 



260 TUNNELING 

omitted. The shield of the Waterloo tunnel is an example of 
this construction. In more treacherous materials, however, not 
only is a diaphragm necessary, but it is also necessary to diminish 
the size of the openings through it, and to provide means for 
closing them entirely. Sometimes only one or two openings are 
left near the bottom of the diaphragm, as in the St. Clair and 
Mersey tunnel shields ; and sometimes a number of smallei 
openings are provided, as in the East River and Hudson Rivei 
tunnel shields. 

In highly treacherous materials subject to sudden and 
violent irruptions of earth from the excavation face, it some- 
times is the case that openings, however small, closed in the 
ordinary manner, are impracticable, and special construction has 
to be adopted to deal with the difficulty. The shields for the 
Mersey and for the Blackwall tunnels are examples of such 
special devices. In the Mersey tunnel a second diaphragm was 
built behind the first, extending from the bottom of the shield 
upward to about half its total height. The aperture in the first 
diaphragm being near the bottom, the space between the second 
and first diaphragms formed a trap to hold the inflowing material. 
The Blackwall tunnel shield, as previously indicated, had its 
front end divided into cells. Ordinarily the face of the excava- 
tion in front of each cell was left open, but where material was 
encountered which irrupted into these cells a special means of 
closing the face was necessary. This consisted of three poling- 
boards or shutters of iron held one above the other against the 
face of the excavation. These shutters were supported by 
means of strong threaded rods passing through nuts fastened 
to the vertical frames, which permitted each shutter to be ad- 
vanced against or withdrawn from the face of the excavation 
independently of the others. Various other constructions have 
been devised to retain the face of the excavation in highly 
treacherous soils, but few of them have been subjected to 
conclusive tests, and they do not therefore justify considera= 
tion. 



Sl'BMAKINK TUNNELlNCr 261 

Rear-End Construction. — By the rear end of the shield is 
meant that portion at the rear of the diaphragm. It may be 
divided into two parts, called respectively the body and the 
tail of the shield. The chief purpose of the body of the shield 
is to furnisli a place for the location of the jacks, pumps, 
motors, etc., employed in manipulating the shield. It also 
serves a purpose in distributing the weight of the shield over 
a large area. To facilitate the passage of the shield around 
curves, or in changing from one grade to another, it is desirable 
to make the body of the shield as short as possible. In the 
^Jersey, Clichy, and Waterloo tunnel shields, and, in fact, 
in most others which have been employed, the shell plates of 
the body have been reinforced by a heavy cast-iron I'ing, within 
and to which are attached the jacks and other apparatus. The 
latest opinion, however, seems to point to the use of brackets 
and beams for strengthening the shell for the purpose named, 
rather than to this heavy cast-iron construction. In the 
Hudson River, St. Clair River, and East River tunnel shields, 
with their long and strongly braced front-end construction to 
carry the jacks, the body of the shield, so to speak, is omitted, 
and the rear-end construction consists simply of the tail plat- 
ing. In the Blackwall shield, the body of the shield shell 
provides the space necessary for the double diaphragms and 
the cells which they inclose. In a general way, it may be 
said that the present tendency of engin'feers is to favor as 
short and as light a Ijody construction as can be secured. 

The tail of the shield serves to support the earth while the 
lining is being erected ; and for this reason it overlaps the 
forward ring of the lining, as shown clearly by most of 
the shields illustrated. To fulfill this purpose, the tail-plates 
should 1x3 perfectly smooth inside and outside, so as to slide 
easily between the outside of the lining plates and the earth, 
and shoidd also Ixi as tliin as practicable, in order not to leave 
a large void behind the lining to be filled in. In soils which 
are fairly stable, the tail construction is often visor-shaped : 



262 



TUNNELING 



that is, the tail-plates overlap the lining only for, say, the roof 
from the springing lines up, as in one of the shields for the 
Clichy tunnel. In unstable materials, the tail-plating ex- 
tends entirely around the shield and excavation. The length 
of the tail-plating is usually sufficient to overlap two rings of 
the lining, but in one of the Clichy tunnel shields it will be 
noticed that it extended over three rings of lining. This 
seemingly considerable space for thin steel plates is made 
possible by the fact that the extreme rear end of the tail 
always rests upon the last completed ring of lining. 

In closing these remarks concerning the rear-end con- 
struction, the accompanying table, prepared by Mr. Raynald 
Legouez, will be of interest, as a general summary of principal 
dimensions of most of the important tunnel shields which have 
been built. The figures in this table have been converted 
from metric to English measure, and some slight variation 
from the exact dimensions necessarily exists. The different 
columns of the table show the diameter, total length, and the 
length of each of the three principal parts into which tunnel 
shields are ordinarily divided in construction as previously 
described : — 



Name of Shield. 



Concorde Siphon . . 
Clichy Siphon . . . 

Mersey 

East River .... 
City and South London 
Glasgow District 
Waterloo and City 
Glasgow Harbor 
Hudson River . 
St. Clair River . 
Clichy Tunnel . 
Clichy Tunnel . 
Blackwall 
Waterloo Station 



Lkngtu in Feet. 



Diameter. 



6 75 

8.39 

0.97 

10.99 

10.99 

12.07 

12.99 

17.25 

19.91 

21.52 

23 7-19.8 

23.8-19.4 

27. t 

24.86 



Tail. 



2.51 
2.51 
5.61 
3.51 
2.65 
2.65 
2.75 
2.75 
4.82 
4.00 
4.00 
7.44 
6.98 
3.34 



Body. 



2.55 
2.55 
2.98 
0.32 

2.82 
2.82 



.98 
.98 
.98 
.98 



2. 
2. 
2. 
2. 
2.98 
11.90 
5.90 
5.51 



Fkont. 



1.16 
1.16 
2.98 
3.67 
1 01 
1.01 
1.24 
1.08 
5.67 
11.25 
6.88 
4.46 
6.59 
1.14 



Total. 



6.67 

6.16 

11.58 

7.51 

6 49 

6.49 

6.98 

8.49 

10.49 

15.25 

17.22 

23.65 

19.48 

10.00 



SUBMAKINE TUNNELING 263: 

Jacks. — The motive power usually employed in driving- 
modern tunnel shields is hydraulic jacks. In some of the 
earlier shields screw-jacks were used, but these soon gave way 
to the more po\>erlul hydraulic device. The manner of 
attaching the hydraulic jacks to the shield is always to fasteni 
the cylinder castings at regular intervals around the inside cf 
the shell, with the piston rods extending backw^ard to a bearings 
against the forward edge of the lining. In the older forms of 
shield, having an interior cast-iron reinforcing ring construc- 
tion, the jack cylinder castings were always attached to this, 
castriron ring; but in many of the later shields constructed 
without this cast-iron reinforcing ring, the cylinder castings- 
are attached to the shell by means of bracket and gusset con- 
nections. The number and size of the jacks employed, and the 
distance apart at which they are spaced, depend upon the size 
of the shield and the character of the material in wdiich it is^ 
designed to work. In stiff and comparatively stable clays, the 
skin friction of the shield is comparatively small, and an ag- 
gregate jack-power of from 4 to 5 tons per square yard of 
the exterior friction surface of the shield has usually been 
found ample. The cylinders are spaced about 5| ft. apart, 
and iiave a working diameter of from 5 to 6 ins., with a. 
water pressure of about 1,000 lbs. per sq. in. In soft, 
sticky material, giving a high skin friction, the aggregate jack- 
power required per square yard of exterior shell surface rises to- 
from 18 to 24 tons; the jacks are spaced about 3 ft. apart; 
and the working cylinder diameter and water pressure are, re- 
spectively, about 6 or 7 ins., and from 4,000 lbs. to 6,000 lbs. 
per sq. in. With these high pressures, power pumps are 
necessary to give the required water pressure ; but where the 
pressure required does not exceed 1,000 lbs. per sq. in., hand 
])umi)S may be, and usually are, employed. The number of 
jacks required de^jends upon the diameter of the shield, and, of 
course, upon the distance apart which they aie placed. In the 
City and South London tunnel shield six ja(;ks were used, and 



264 



TUNNELING 



111 the Blackwall shield 24 were used. The mechanical 
■construction of the jacks for tunnel shields presents no features 
out of the usual lines of such devices used elsewhere. The 
jacks used on the East River tunnel shield are shown by Fig. 
118, and those for the St. Clair River tunnel by Fig. 133. 




Part Transverse Section. Longitudinal Section. 

Fig, 133. — Cast Iron Lining, St. Clair River Tunnel. 

Two general methods are employed for transmitting the 
thrust of the piston rods against the tunnel lining. The 
object sought in each is to distribute the thrust in such a 
manner that there is 'no danger of bending the thin front flange 
of the forward lining ring. In English practice the plan 



SUBMARINE TUNNELING 265 

usuall}' adopted, is to attach a shoe or bearing casting to the 
end of the piston rod, which A\ill distribute the pressure over 
a considerable area. An example of this construction is the 
shield for the City and South London tunnel. In the East 
River and St. Clair River tunnels, built in America, the tail of 
the piston rod is so constructed that the thrust is carried 
directly to the shell of the lining. 

LINING. 

Either iron or masonry may be used for lining shield-driven 
tunnels but present practice is almost universally in favor of 
iron lining. As usually built, iron lining consists of a series of 
successive cast iron rings, the abutting edges of which are pro- 
vided with flanges. These flanges are connected by means of 
butts, the joints being packed with thin strips of w^ood, oakum, 
cement, or some other material to make them water-tight. 
Each lining ring is made up of four or more segments, which 
are pro\ided with flanges for bolted connections similar to 
those fastening the successive rings. Generally the crown seg- 
ment is made considerably shorter than those forming the sides 
and bottom of the ring. The erection of the iron segments 
forming the successive rings of the lining may be done by hand 
in tunnels of small diameter where the weights to be handled 
are comparatively light, but in tunnels of large size special 
cranes attached to the shield or carried by the finished lining 
are employed. The construction of the iron lining for the East 
River tunnel is illustrated in Chapter XIX., and that for the 
St. Clair River tunnel is shown by Fig. 133. 



266 TUNNELING 



CHAPTER XXII. 

ACCIDENTS AND REPAIRS IN TUNNELS DURING 
AND AFTER CONSTRUCTION. 



In the excavation of tunnels it often happens that the dis- 
turbance of the equilibrium of the surrounding material by the 
excavation develops forces of such intensity that the timbering 
or lining is crushed and the tunnel destroyed. To provide 
against accidents of this kind in a theoretically perfect manner 
would require the engineer to have an accurate knowledge of 
the character, direction and intensity of the forces developed, 
and this is practically impossible, since all of these factors differ 
with the nature and structure of the material penetrated. The 
best that can be done, therefore, is to determine the general 
character and structure of the material penetrated, as fully as 
practicable, by means of borings and geological surveys, and 
then to employ timbering and masonry of such dimensions and 
character as have withstood successfully the pressures devel- 
oped in previous tunnels excavated through similar material. 
If, despite these precautions, accidents occur, the engineer is 
compelled to devise methods of checking and repairing them, 
and it is the purpose of this chapter to point out briefly the 
most common kinds of accidents, their causes, and the usual 
methods of repairing them. 

Accidents During Construction. — Accidents may happen both 
during or after construction, but it is during construction, when 
the equilibrium of the surrounding material is first disturbed, 
and when the only support of the pressures developed is the 
timber strutting that they most commonly occur. 



ACCIDENTS AND KEPAIRS IN TUNNELS 267 

Causes of Collapse. — Collapse in tunnels may be caused : (1) 
by the weight of the earth overhead, which is left unsupported 
by the excavation ; (2) by defective or insufficient strutting ; 
and (3) by defective or weak masonry. 

(1) The danger of collapse of the roof of the excavation is 
influenced by several conditions. One of these is the method 
of excavation adopted. It is obvious that the larger the 
volume of the supporting earth is, which is removed, tie 
greater will be the tendency of the roof to fall, and the more 
intense will be the pressures which the strutting will be called 
upon to support. Thus the English and Austrian methods of 
tunneling, where the full section is excavated before any of the 
lining is placed, and where, as the consequence, the strutting 
has to sustain all of the pressures, present more likelihood of 
the roof caving in than any of the other common methods. 

The character and structure of the material penetrated also 
influence the danger of a collapse. A loose soil with little 
cohesion is of course more likely to cave than one which is 
more stable. Rock where strata are horizontal, or which is 
seamy and fissured, is more likely to break down under the roof 
pressures than one with vertical strata and of homogeneous 
structure. Soft sod containing boulders whose weight develops 
local stresses in the roof timbering is likely to be more danger- 
ous than one which is more homogeneous. A factor which 
greatly increases the danger of collapse, especially in soft soils, 
is the presence of water. This element often changes a soil 
which is comparatively stable, when dry, into one which is 
highly unstable and treacherous. The liability of the material 
to disintegration by atmospheric influences and various other 
conditions, which will occur to the reader, may influence its 
stability to a dangerous extent, and result in collapse. 

(^2) Collapse is often the result of using defective or insuf- 
ficient strutting. Of course, in one sense, any strutting which 
fails under the pressures deveh)i)ed, however ononnous they 
may be, can be said to be insuflicient, but as used heie the term 



268 TUNNELING 

means a strutting with an insufficient factor of safety to meet 
probable increases or variations in pressure. Insufficient strut- 
ting may be due to the use of too light timbers, to the spacing 
of the roof timbers too far apart, to the yielding of the founda- 
tions, to insufficient bearing surface at the joints, etc. Collapse 
is often caused by the premature removal of the strutting dur- 
ing the construction of the masonry. The masons, to secure 
more free space in which to work, are very likely, unless 
watched, to remove too many of the timbers and seriously 
weaken the strutting. 

(3) The third cause of collapse is badly built masonry. 
Poor masonry may be due to the use of defective stone or brick, 
to the thinness of the lining, to poor mortar, to weak centers 
which allow the arch to become distorted during construction, 
to poor bonding of the stone or bricks, to the premature 
removal of the centers, to driving some of the roof timbers 
inside it, etc. 

Prevention of Collapse. — Tunnels very seldom collapse with- 
out giving some previous warning of the possible failure, and 
also of the manner in which the failure is likely to occur. 
From these indications the engineer is often able to foresee the 
nature of the danger and take steps to check it. The danger 
may occur either during excavation or after the lining is built. 
During excavation the danger of collapse is indicated before- 
hand by the partial crushing or deflection of the strutting tim- 
bers. If the timbers are too light or the bearing surfaces are 
too small, crushing takes place where the pressures are the 
greatest, and the timbers bend, burst, or crack in places, and the 
joints op3n in other places. The remedy in such cases is to in- 
sert additional timbers to strengthen the weak points, or it may 
be necessary to construct a double strutting throughout. 
When the distance spanned by the roof timbers is too great, 
failure is generally indicated by the excessive deflection of 
these timbers, and this may often be remedied by inserting 
intermediate struts or props. In some respects the best remedy 



ACCIDENTS AND REPAIRS IN TUNNELS 269 

under any of these conditions is to construct the masonry as 
soon as possible. 

When collapse is likely to occur after the masonry is com- 
pleted, its probability is generally indicated by the cracking 
and distortion of the lining. A study of the cause is quite 
likely to show that it is the percolation of water through the 
material surrounding the lining which causes cavities behind 
the lining in some places, and an increase of the pressures in 
other places. When it is certain that this water comes from 
the surface streams above, these streams may often be diverted 
or have their beds lined with concrete to prevent further perco- 
lation. When percolating water is not the cause of the trouble, 
a usually efficient remedy is to sink a shaft over the weak point, 
and refill it with material of more stable character. These, 
and the remedies previously suggested, are designed to prevent 
failure without resorting to reconstruction. When they or 
similar means prove insufficient, reconstruction or repairs have 
to be resorted to. 

Repairing Failures. — Tunnels may collapse in several ways : 
(1) The front and sides of the excavation may cave in; (2) 
the floor or bottom may bulge or sink; (3) the roof may fall 
in ; (4) the material above the entrances may slide and f^ll 
them up. 

(1) One of the most common accidents is the caving of the 
front and sides of the excavation. This may often be prevented 
by taking care that the face of the excavation follows the natu- 
ral slope of the material instead of being more or less nearly 
vertical. When, however, caving does occur it may usually 
be repaired by removing the fallen material, strongly shoring 
the cavity, and filling in })ehiiid with stone, timl)er, or fascines. 

(2) The bulging or rising of the bottom of the tunnel may 
usually Ije considered as a consequence of the squeezing together 
of the side walls. It usually occurs in very loose soils, and is 
chiefly important from the fact that the reconstruction of the 
side walls is made necessary. The sinking of the tunnel hot- 



270 TUNNELING 

torn is a more serious occurrence. It seldom happens unless 
there is a cavity beneath the floor, due either to natural causes 
or to the fact that mining operations have gone on in the hill 
or mountain penetrated by the tunnel. When the bottom of 
the tunnel sinks, three cases may be considered : (a) when the 
sinking is limited to the middle of the tunnel floor ; (5) when 
only a portion of the foundation masonry is affected ; and, (c) 
when the entire lining is disturbed. In the first case repairs 
are easily made by filling in the cavity with new material. In 
the second case the unimpaired portion of the masonry is tem- 
porarily supported by shoring while the injured portion is re- 
moved and rebuilt on a firm foundation. The remaining cavity 
is then filled. In the case of the complete failure of the lining, 
the method of repairing employed when the roof falls, and 
described below, is usually adopted. 

(3) The most dangerous of all failures is the falling of the 
tunnel roof. In such casualties two cases may be considered : 
(a) When the falling mass completely fills the tunnel section, 
and (^) when it fills only a portion of the section. 

When the whole section is filled by the fallen material, the 
problem may be considered as the excavation of a new tunnel 
of short length inside the old tunnel, and under rather more 
difficult conditions. The first task, particularly if men have 
been imprisoned behind the fallen material, is to open com- 
munication through it between the two uninjured portions of 
the tunnel. It is advisable to do this even when there is no 
danger to life because of imprisoned workmen, since it enables 
the work of repairing to be conducted from both directions. 
The excavation of a passageway through the fallen material 
is rendered difficult, both because the fallen material is of an 
unstable character, and also because it is usually filled with the 
lining masonry, timbering, etc. When, therefore, the accident 
has happened before the full section of the original material 
has been removed, the first heajcling or drift is driven through 
this original material rather than through the fallen debris. 



ACC1DE^'TS AND llEl»AlliS IN TUNNKLS 



271 



Any of the regular soft^ground methods of tunneling may be 
employed, but it is usually better to select one which allows 
the masonry to be built with as little excavation as possible at 
first. For this reason the German method of tunneling is par- 
ticularly suited to repair work of this nature. The Belgian 
method may also be used to advantage, particularly when the 
caving extends to the surface of the ground above, and the 
upper portion of the debris is, therefore, practically the same 
material as that through which the original tunnel was driven. 
The greatest defect of the Belgian method for making repairs 
is that the roof arch is supported by a rather unstable mass of 




Fig. 1;M. — Tunneling through Caved Material by Heading. 

mingled earth, stone, and timber, which constitutes the bottom 
layer of the fallen material. The method of strutting the w^ork 
when the German or I>elgian method is used is shown by Fig. 
184. It sometimes happens that the fallen debris is so un- 
stable tliat it will not carry safely tlie arch masonry in the 
Belgian method or the strutting in the German method, and in 
these cases one of the full-section methods of excavation is 
usually adopted. The nature of the strutting employed is 
shown by Fig. 18o. When the section has been opened and 
the new masonry built, great care should be taken to fill the 
cavity behind the masonry with timber or stone ; and should 



272 



TUNNELING 



the disturbance reach to the ground surface it is often a good 
plan to sink a shaft through the disturbed material, and fill it 
with more stable material. 




Fig. 135. — Tunneling through Caved Material by Drifts. 

When the fallen debris fills only a part of the section, the 
first thing to provide against is the occurrence of any further 
caving ; and this is usually done by building a protecting roof 
above the line of the future roof masonry. Figs. 136 and 137 





Figs. 136 and 137. —Filling in Roof Cavity Formed by Falling Material. 



show two methods of constructing this temporary roof, which 
it will be noticed is filled above with cordwood packing. As 
soon as the temporary roof is completed, the lining masonry is 
constructed. 



ACCIDENTS AND KEPAIKS IN TUNNELS 



273 



(4) Landslides which close the tunnel entrance are repaired 
in a variety of ways. Fig. 138 shows a common method of 
preventing the extension of a landslide which has been started 




Fig. 138. — Timbering to Prevent Landslides at Portal. 

by the excavation for the entrance masonry. Fig. 139 shows a. 
method often adopted when the slope is quite flat and the 
amount of sliding material is small. It consists essentially of 
removing the fallen material and building a new portal farther 
back ; that is, the open 
cut is extended and the 
tunnel is shortened. 
When the amount of 
the sliding material is 
very large, the contrary 
practice of lengthening 
the tunnel and shorten- 
ing the open cut, as 
shown by Fig. 140, 
may be adopted. 

Accidents After Construction. — Accidents after the comple- 
tion of the tunnel may ha divided into two classes : first, 
those which entirely obstruct the passage of trains, of which the 
collapse of the roof is the most common; and second, those which 
allow traffic to be continued while the repairs are being made. 




Fig. 139. —Shortening Tunnel Crushed by Landslide 

at Portal. 



274 TUNNELING 

such as the bulging inward of a portion of the lining without 
total collapse. In the first case the first duty of the engineer 
is to open communication through the fallen debris, so 
that passengers at least may be transferred from one part of the 
tunnel to the other and proceed on their way. This is done 
by driving a heading, and strongly timbering it to serve as a 
passageway. If the tunnel is single tracked this heading is 
afterwards enlarged until the whole section is opened. In 
double-track tunnels the method generally adopted is to open 
first one side of the section and timber it strongly, so as to clear 
one track for traffic. While the trains are run- 
ning through this temporary passageway the 
other half of the section is opened and re- 
paired ; the traffic is then shifted to the 
new permanent track, and the temporary 
structure first employed is replaced 
with a permanent lining. 
When the accident is such 
that the repairs can be 
made without ob- 
structing traffic en- 
tirely, Var 1 O U S -p^^ 140. — Extending Tunnel through Landslide at Portal. 

modes of procedure 

are followed. In all cases great care has to be exercised to 
prevent accident to the trains and to the tunnel workmen. 
The work should be done in small sections so as to disturb as 
little as possible the already troubled equilibrium of the soil ; 
the strutting should be placed so as to give ample clearing 
space to passing trains, and the trains themselves should be run 
at slow speeds past the site of the repairs. To illustrate the 
two kinds of accidents and the methods of repairing them, 
which have been mentioned, the accidents at the Giovi tunnel 
in Italy and at the Chattanooga tunnel in America have been 
selected. 

Giovi Tunnel Accident. — In September, 1869, at a point about 




ACCIDENTS AND llEPAIKS IN TUNNELS 275 

220 ft. from the south portal of the Giovi tunnel, a disturbance 
of the masonry lining for a length of about 52 ft. was observed. 
Accurate measurements showed that the lining was not sym- 
metrical with respect to the vertical axis of the sectional profile. 
It was concluded that owing to some disturbance of the sur- 
rounding soil unsynnnetrical vertical and lateral pressures were 
acting on the masonry. Close Avatch was kept of the dis- 
torted masonry, which for some time remained unchanged 
in 2)osition. In 1872, however, new crevices were observed 
to have developed, and shortly afterwards, in January, 1873, 
t!ie injured portion of the masonry caved in, obstructing 
the whole tunnel section. The fallen material consisted 
chiefly of clay in a nearly plastic state. The surface of the 
ground above was observed to have settled. Investigation 
showed also that the cause of the caving was the percolation of 
water from a nearby creek. The water had soaked the ground, 
and decreased its stability to such an extent that the masonry 
lining was unable to withstand the increased vertical and lateral 
pressures. 

The mode of procedure decided upon for repairing the 
damage was : (1) To open at least one track for the temporary 
accommodation of traffic ; (2) To remove permanently the causes 
which had produced the collapse ; (3) To build a new and 
much stronger lining. Close to the western side wall, which 
was still standing, the debris was removed, and the opening 
strongly strutted in order to allow the laying of a single 
track to reestablish communication. At the same time a shaft 
was sunk from the surface above the caved portion of the tunnel, 
for the double purpose of facilitating the removal of the 
fallen material and of affording ventilation. The depth of the 
sui-face above the tunnel Avas 41.6 ft., which made the construc- 
tion of the shaft a cf)inparatively easy matter. The shaft itself 
was C)\ ft. wide and 18 ft. long, with its longer dimensions parallel 
to the tunnel, and it was lined with a rectangular horizontal 
frame and vertical-poling board construction. After tern- 



276 TUNNELING 

porary communication had been opened on the western track of 
the tunnel, the remainder of the fallen earth was removed and 
the excavation strutted. The new masonry lining was then 
built. 

To remove permanently the cause of the cave-in, which was 
the percolation of water from a close-by stream, this stream was 
diverted to a new channel constructed with a concrete bed and 
side walls. 

The failure of the original lining occurred by cracks develop- 
ing at the crown, haunches, and springing lines. The new lining 
was made considerably thicker than the original lining, and at 
the points where failure had first occurred in the original arch 
cut-stone voussoirs were inserted in the brickwork of the new 
arch as described in Chapter XIII. 

Chattanooga Tunnel. — The Western & Atlantic Ry. passes 
through the Chattanooga mountains by means of a single-track 
tunnel 1,477 ft. long, constructed in 1848-49. The lining con- 
sisted of a brickwork roof arch and stone masonry side walls. 
After the tunnel had been opened to traffic, this lining bulged 
inward at places, contracting the tunnel section to such an ex- 
tent that it was decided to reconstruct the distorted portions. 
After careful surveys and calculations had been made, it was 
decided to take down and reconstruct about 170 ft. of the 
lining. 

Owing to contracted space in the tunnel, it was necessary 
to remove all men, tools, and material, whenever trains were 
to pass through ; and in order to do this a work-train of 
three cars was fitted up with necessary scaffolds, and supplied 
with gasoline torches for lighting purposes. Mortar was mixed 
on the cars, and all material remained on them until used. 
Debris torn out of the old wall was loaded on the cars, and 
hauled to the waste dump. A siding was built near the West 
end of the tunnel for the use of this train, and a telephone sys- 
tem was installed between the entrances and the working-train. 
On account of the contracted working-space and the greater 



ACCIDENTS AND KEPAIKS IN TUNNELS 277 

ease with which brick could be handled, it was decided to re- 
build the walls out of brick instead of stone. 

In tearing out the old wall a hole was first cut through the 
three bottom courses of the arch and gradually widened. When 
the opening became four or five feet long, a small jack was 
placed near the center of it and brought to a bearing against 
the arch to sustain it. After cutting the opening to a length 
of from 7 to 10 ft. depending on the stability of the earth 
backing, the jack was removed and a piece of 8x16 in. timber 
placed under the arch and brought up to a bearing with jacks. 
One end of the timber rested on the old wall, the other on a seat 
built into the adjoining section of new wall. Wedges were 
then driven under the ends of timber and the jacks removed. 
With this timber in place, the old wall could be taken down 
with ease, the only trouble being that small stones and earth 
fell in from above and behind the arch. This w^as obviated 
by placing a 2 in. plank across the opening and just back of 
the 8x16 in. timber. At several points, however, the earth 
backing was saturated with water, and it became necessary to 
put in lagging as the old wall was removed. This timbering 
would be taken out as the new work was built up. 

A suitable foundation for the new wall was secured at a 
depth from 2 to 4 ft., and a concrete footing was used. The 
section of the new wall was then built up as near as possible to 
the 8x16 in. timber; the timber was then removed and the 
new wall built up and keyed under the arch. 

The new wall had a minimum width of 2^ ft. at the top, 
and 4 ft. at the base of rail, and was provided with weep holes 
at intervals. To facilitate matters, work was carried on simul- 
taneously at two or three different places, the intention being 
to get one place torn out and ready for the bricklayers by the 
time they completed a section of the new wall at another 
place. 

In rebuilding the arch, sections extending from the spring- 
ing line up as far as was necessary to obtain the desired clear- 



278 TUNNELING 

ance, and from 2J to 4 ft. in length, were removed. Near the 
sides, the earth above the arch was a stiff clay, which was self- 
sustaining; but near the center there occurred a stratum of 
gravel and clay saturated with water. This gave considerable 
trouble, falling through almost continuously until timbering 
could be placed. One end of this timber rested on the old 
arch, the other on the adjoining section of the new work. As 
the new work was to be set 6 to 13 ins. back from the old, it 
was necessary to block up this distance on top of the old arch, 
to carry the end of the lagging timber, in order that the timber 
should be clear of the new arch. 

Owing to the small clearance between the car roof and the 
arch, a special form of centering was required, one that would 
occupy as small space as possible. Bar iron 1 in. thick, 4 ins. 
wide, and 20 ft. long was carved to a radius of 6i ft., and on 
the underside of this was riveted a 6-in. plate k in thick. This 
plate projected 1 in. on the sides of the centering, and carried 
the ends of the 1 in. boards used for lagging. The rivets were 
counter-sunk on the outside of the centering to present a smooth 
surface next the arch. 

In keying up a section of the new work, a space about 18 ins. 
square had to be left open for the use of the workmen. As 
soon as the next section had been torn out, this space \^as built 
up. In building up the last section, this space had to be filled 
from below, which proved to be a tedious undertaking. The 
opening was gradually reduced to a size of 10 x 18 in., and the 
top ring then completed and keyed up, the adhesion of mortar 
holding the bricks in place until the key could be driven home. 
The next ring was treated in a similar manner, and so on to the 
face ring. Altogether 412 lin. ft. of the walls and 178 lin. ft. 
of the arch were taken down and rebuilt, amounting in all to 
607 cu. yds. of masonry at the total cost of |7,440, or about 
112.25 per cu. yds. 

The regular trains arrived so frequently at the tunnel that 
slightly over two hours was the longest working-time between 



ACCIDENTS AND REPAIRS IN TUNNELS 279 

any two trains, and usually less than one hour at a time was all 
that it could be worked. In addition to the regular trains, a 
large number of extra trains, moving troops, had to be accom- 
modated. Work was in progress eight months, and during that 
time there was no delay to a passenger train. The repairs were 
completed in August, 1899. The work was under the direction 
of Mr. W. H. Whorley, engineer of the Western & Atlantic 
R. R., and foreman of construction, A. H. Richards. A recent 
examination failed to reveal any sign of settlement cracks at the 
junction points of the new and old work. 



280 TUNNELING 



CHAPTER XXIII. 

RELINING TIMBER LINED TUNNELS WITH 
MASONRY. 



The original construction of many American railway tunnels 
^th a timber lining to reduce the cost and hasten the work has 
made it necessary to reline them, as time has passed, with some 
more permanent material. In most cases the work of removing 
the old lining and replacing it with the new masonry has had 
to be done without interfering with the running of trains, and a 
number of ingenious methods have been developed by engineers 
for accomplishing this task. Three of these methods which 
have been employed, respectively, in relining the Boulder 
tunnel on the Montana Central Ry., in Montana, the Mullan 
tunnel on the Northern Pacific Ry., in Montana, and the Little 
Tom tunnel on the Norfolk & Western R. R., in Virginia, have 
heen selected as fairly representative of this class of tunnel 
work. 

Boulder Tunnel. — This tunnel penetrates a spur of the main 
range of the Rocky Mountains, at an elevation at the summit 
of grade of 5,454 ft., and is B,112 ft. in length. Its alinement is 
a tangent, with the exception of 150 ft. of 30' curve at the 
north end. The material penetrated is blue trap-rock with 
seams for 4,950 ft. from the north end, and syenitic boulders 
with the intervening spaces filled with disintegrated material 
for the remaining 1,160 ft. The dimensions and character of 
the old timber lining and of the new masonry lining replacing 
it are shown in Figs. 141 and 142. 

The form of masonry adopted consisted of coarse rubble side 
walls of granite, 13 ft. 8 ins. high, and generally 20 ins. thick. 



RELINING TEMBEK-LINED TUNNELS WITH MASONRY 



281 



with a full center circular arch of four rings of brick laid in 
rowlock form. When greater strength was needed the thick- 
ness of the side walls was increased to 30 ins. and that of the 
arch to six rings of brick. 

The first plan adopted in putting in the masonry was to 
remove all the timbering ; but owing to the large number of 
falls and slides this was abandoned, and the plan followed was to 
leave in the three roof segments of the timbering with the over- 
lying cord- wood packing and debris. In carrying on the work 
the first step was to remove the side timbers. This was done 
by supporting the roof timbers, as shown in Fig. 141 ; that is, 
the first and fourth arch rib of an 8-ft. section containing four 




Longitudinal Section. 

Cress Section. 

Fifis. 141 and 142. — Relining Timber-Lined Tunnel. 



*'^^' 



arch ribs were supported by temporary posts. The intermedi- 
ate arch ribs were supported against the downward pressure by 
6 X 6 in. timbers, extending from the side ribs near the tops 
of the temporary posts to the opposite sides of the intermediate 
roof segments, as shown in the longitudinal section. Fig. 142. 
To resist the pressure from the sides, 4x6 in. braces were 
placed across the tunnel from near the center of the intermedi- 
ate segments to the upper ends of the hip segments, as shown 
in the cross-section, Fig. 141. The hip segments were then 
sawed off below the notch, and the side timbering removed and 
the masonry built. 

The stone was conveyed into the tunnel on flat cars, and laid 
by means of small derricks located on the oai-s. Two derricks 



282 



TUNNELING 



were used, one for each side wall, and the work on both walls 
was carried on simultaneously. 

The arch was built upon a centering, the ribs of which were 
5i ins. less in diameter than the distance between the side 
walls, so as to permit the use of 2| ins. lagging. Each center 
had three ribs, made in 1-in. or 2-in. board segments, 10 ins. thick 
and 14 ins. deep. These ribs were mounted on frames, which 
followed the opposite walls, and were 4 ft. apart, making the 
total length of the center out to out about 9 ft. The frames, 
upon which the ribs were supported, are shown in Fig. 143. 
As will be seen, they were mounted on doUys to enable the 
center to be moved from one section to another. Jacks were 

used to raise and lower 
the center into its proper 
position. 

The arch was built up 
from the springing lines 
on both sides at the same 
time, four masons being 
employed. The rings 
were built beginning with 
the intrados, which was 
brought up, say, a dis- 
tance of about 2 ft. from the springing line. Then the back of 
the ring was well plastered with from | in. to \ in. of mortar, 
and the second ring brought up to the same height and 
plastered on the back, and so on until the last ring was laid. 
After bringing the full width of the arch up some distance, 
new laggings were placed on the ribs for an additional height 
of 2 ft. and the same process was repeated. All the space 
between the extrados of the masonry arch and the old lining 
was compactly filled with dry rubble. When high enough 
so that the hip segments had a foot or more bearing on the 
masonry the segments were securely wedged and blocked up 
against the brickwork, and the longitudinal 4 X 6 in. timbers 




Cross SecHon. Longitudmal Section. 

Fig. 143. — Relining Timber-Lined Tunnel, 
Great Northern Ry. 



RELlNtNG TIMBER-LINED TUNNELS WITH MASONRY 



28a 



removed. The remaining space was now clear for completion 
of the arch, and both sides were brought up until there was 
not sufficient space for four masons to work, when the keying 
was completed by two masons beginning at the completed and 
working back toward the toothed end. The brickwork was 
built from the top of a staging-car. 

In a few instances where slides occurred after the removal 
of the slide timbering, the method of re timbering the tunnel 
shown in Fig. 144 was adopted. Two side drifts were first 
run 2i ft. wide by 4 ft. high, and the plate timbers placed in 
position and blocked. Cross drifts were then run, and the roof 
segments placed, and the core down to the level of the bottoms 
of the side drifts taken 
out. The lower wall 
plates were then placed 
and the hip segments 
inserted. The bench 
was then taken down 
by degrees, the side 
plates being held by 
jacks, and the posts 
placed one at a time. 
As the masonry at the 
points where slides occur consists of 30-in. walls and six-ring 
arch, the timbering was 22 ft. wide in the clear, with other 
dimensions as shown in Fig. 144. 

Only a single crew of brick and stone masons was employed. 
In order to prepare the sections for these masons it was 
necessary to have timber and tiimming crews at work through- 
out the whole day of 24 hours, so that an engine and two train 
crews were in constant attendance. The single mason crews 
were able to complete 8 ft. of side wall and arch in 24 hours. 
The number of men actually employed at the tunnel was 85. 
This included electric-liorht maintenance, and all other labor 
pertaining to the work. The tunnel was lighted by an Edison 




■7.'ei} 
Cross Section. Longitudinal Section. 

Fig. 144. — Ilelining Timber-Lined Tunnel, 
Great Northern Ry. 



234 



TUNNELING 




WthmnP/afes Wrthout Wall Plates. 

Old Timber Sections 



dynamo of 20 arc light capacity, one arc light being placed on 
each side of the tunnel at all working-places. Each lamp 
carried a coil of wire 20 or 30 ft. long to allow it to be shifted 
from place to place without delay. 

Mullan Tunnel. — This tunnel is 3,850 ft. long, and crosses 
the main range of the Rocky Mountains, about 20 miks 
west of Helena, Mont. The tunnel is on a tangent throughout, 
and has a grade of 20 % falling toward the east. The summit 

of the grade, west of the tun- 
nel, is 5,548 ft. above sea 
level, and the mountain above 
the line of the tunnel rises 
to an elevation of 5,855 ft. 
Owing to the treacherous 
nature of the material through 
which the tunnel passed, it 
had been a constant menace 
to trafhc ever since its con- 
struction in 1883, and numer- 
ous delays to trains had been 
caused by the falls of rock 
and fires in the timber lin- 
ing. For these reasons it was 
finally decided to build a per- 
manent masonry lining, and 
work on this was begun in July, 1892. 

The original timbering consisted of sets spaced 4 ft. apart 
c. to c., with 12 X 12 in. posts supporting wall plates, and a 
five-segment arch of 12 x 12 in. timbers joined by li-in. 
dowels. The arch was covered with 4-in. lagging, and the 
space between this and the roof was filled with cordwood. 
Except where the width had been reduced by timbering placed 
inside the original timbering to increase the strength, the clear 
width was 16 ft., and the clear height 20 ft. above the top of 
the rail. Fig. 145 shows the timbering and also the form 




Permanent Work. 



Averrjqe , 



^^^^'^ 



Fig. 145. — Reliuiiig Timber Lined Tunnel, 
Great Northern Ry. 



KELINING TIMBEK-LINED TUNNELS WITH MASONKY 285 



of masonry lining adopted. The side walls are of concrete and 
the arch of brick. This new masonry, of course, required the 
removal of all the original timbering. The manner of doing 
this work is as follows: A T-ft section, A B, Fig, 146, was first 
prepared by lemoving one post and supporting the arch by 
struts, iS S. After clearing away any backing, and excavating for 
the foundation of the side wall, two temporary posts, J^ i^, Avere 
set up, and fastened by hook bolts. Fig. 146, L, and a lagging 
was built to form a mold for the concrete. Several of these 
T-ft. sections were prepared at a time, each two being sepa- 
rated by a 5-ft. section of timbering. 




i ^1 m ^ 



^m 



Laggimf 



iiiL 



fc^ 



With Wall.Plaie . Without WoU. Plate, 

Section .with Concrete Car. Longitudinal Section. 

Fig. 146. — Construction of Centering Mullan Tunnel. 

The mortar car was then run along, and enough mortar 
(1 cement to 3 sand) was run by the chute into each section 
to make an 8-in. layer of concrete. As the car passed along 
to each section, broken stone was shoveled into the last preced- 
ing section until all the mortar was taken up. The walls were 
thus built up in 8-in. layers, and became hard enough to suj> 
port the arches in about 10 to 14 days. The arches were then 
allowed to rest on tlie wall, and the posts of the remaining 5-ft. 
sections were removed, and the concrete wall built up in the 
same way as before. 



286 



TUNNELING 



The average progress per working-day was 30 ft. of side 
wall, or about 45 cu. yds. ; and the average cost, including all 
work required in removing the timber work, train service, lights 
and tools, engineering and superintendence, and interest on 
plant, was 18 per cubic yard. 

The centering used for putting in the brick arches is shown 
in Fig. 147. From 3 ft. to 9 ft. of arch was put in at a time, 
the length depending upon the nature of the ground. To re- 
move the old timber arch, one of the segments was partly sawed 
through; and then a small charge of giant powder was exploded 

in it, the resulting debris, 
cordwood, rock, etc., being 
caught by a platform car ex- 
tending underneath. From 
this car the debris was re- 
moved to another car, which 
conveyed it out of the tunnel. 
The center was then placed 
and the brickwork begun, the 
cement car shown in Fig. 146 
being used for mixing the 
mortar. The size of the 
bricks used was 2i + 2i -|- 9 
ins., four rings making a 20- 
in. arch and giving 1.62 cu. yds. of masonry in the arch per 
lin. ft. of tunnel. The l)ricks were laid in rowlock bond, two 
gangs, of three bricklayers and six helpers each, laying about 12 
lin. ft. per day. The brickwork cost about |17 per cu. yd. 
The total cost of the new lining averaged about $50 per lin. ft. 
Little Tom Tunnel. — The tunnel has a total length of 1,902 
ft., but only 1,410 ft. of it were originally lined with timber. 
This old timber lining consists of bents spaced 3 ft. apart, and 
located as shown by the dotted lines in the cross-section. Fig. 
148. Instead of renewing this timber, it was decided to replace 
it with a brick lining. Although the tunnel was constructed 



1 


1 


3 P 

1 , . . ^i 


> 


fl 


?V<,''°:' E 


sV ■ \* 


jl 


'^ 






: 



n^ 



Fig. 147. — Centering Mullan Tunnel. 



RELLNIXG TIMBER-LINED TUNNELS WITH MASON KV 287 




•288 



TUNNELl^iG 



through rock, this rock is of a seamy character, and in some 
portions of the tunnel it disintegrates on exposure to the air. 
Id removing the timber to make place for the new lining some 
of the roof was found close to the lagging, but often also con- 
siderable sections showed breakages in the roof extending to a 
height varying from 1 ft. to 12 ft. above the upper side of the 
timbering. This dangerous condition of the roof made it neces- 




FlG. 149. — Kelining Timber-Lined Tunnel, Norfolk and Western Ky. 



sary that only a small section of the timber lining should be 
removed at one time. It made it necessary, also, that the brick 
arch should be built quickly to close this opening, and finally 
that all details of centers, etc., should be arranged so as to 
furnish ample clearance to trains. The accompanying illustra- 
tions show the solution of the problem which was arrived at. 
Referring to the transverse and longitudinal sections shown 



KEL1NI>;G TIMBEK-LINED TUNNELS WITH MASONRY 289 

by Fig. 148, it will be seen that two side trestles were built to 
carry au adjustable centering for the roof arch. Two sections- 
of these trestles and centerings were used alternately, one being- 
carried ahead and set up to remove the timbering while the- 
masons were at work on the other. The manner of setting up. 
and adjusting the trestles and centerings is shown by Fig. 148. 
and also by Fig. 149, which is an enlarged detail drawing of 
the set screw and rollers for the centering ribs. The following 
is the bill of material required for one set of trestles and one 
center : 

Trestles : 

Caps and sills -. . 8 pieces 8x8 ins. x 20 ft. 

Posts 18 " 8 X 8 " X 11 " 

Braces 16 " 6 x 4 " x 7 '«■ 

Centerings : 

Ribs 27 " 2 X 18 " X 7 "■ 

Bracing 12 " 2 x 8 " x 7 '^ 

Support to crown lagging 2 " 6 x 6 " x 10 "■ 

Crown lagging 20 " 3 x 6 " x 2 "■ 

Side lagging 30 " 3 x 6 " x 10 '^ 

Side strips 2 " 2 x 12 " x 9 '' 

Blocking for rollers 1 " 6 x 8 " x 12 " 

6 screw and roller castings complete with bolts and lever ; 114 
bolts l-ins. in diameter ; 7i U. H. hexagonalnut and 2 cast washers 
each. 

AVitli this arrangement the progress made per day varied 
from 2 lin. ft. to 3 lin. ft. of lining complete. By work com- 
plete is meant the entire lining, including stone packing between 
the brickwork and the rock. On Feb. 23, 1900, 363 ft. of lin- 
ing had been completed, at a cost of 133.50 jier lin. ft. This 
cost includes the cost of removing the old timber, the loose rock 
above it, and all other work whatsoever. 



290 TUNNELING 



CHAPTER XXIV. 

THE VENTILATION AND LIGHTING OF TUN- 
NELS DURING CONSTRUCTION. 



VENTILATION. 

In long tunnels, especially when excavated in hard rock, 
proper ventilation is of great importance, because the air cannot 
be easily renewed, and the amount of oxygen consumed by 
miners' horses and lamps during construction is very large. 
The gases produced by blasting also tend to fill the head of ex- 
cavation with foul air. Pure atmospheric air contains about 
21 % of oxygen and only 0.04 % of carbonic acid; when the 
latter gas reaches 0.1 %, the fact is indicated by the bad odor; 
at 0.3 % the air is considered foul, and when it reaches 0.5 % it 
is dangerous. It is generally admitted that the standard of 
purity of the air is when it contains 0.08 % of carbonic acid. 

A large quantity of carbonic acid in the air is easily detected 
by observing the lamps, which then give out a dim red light 
and smoke perceptibly ; the workmen also suffer from headache 
and pains in the eyes, and breathe with difficulty. Naturally, 
miners cannot easily work in foul air and, therefore, make very 
slow progress. It is, therefore, to the interest of the engineer to 
afford good ventilation, not only because of his duty to care for 
the safety and health of his men, but also for reasons of econ- 
omy, so that the men may work with the greatest possible ease, 
thus assuring the rapid progress of the work. 

It would be impossible to change completely the atmosphere 
inside a tunnel, as the gases developed from blasting will pene- 
trate into all the cavities and gather there, but the fresh air 



VENTILATION AND LI(;HTIN(r DURING CONSTRUCTION 291 

carried inside by ventilation has a very small percentage of car- 
bonic acid, mixes with that which contains a greater quantity, 
and dilutes it until the air reaches the standard of purity. We 
have not here considered the gases developed from the decom- 
position of carboniferous and sulphuric rocks, which may be 
met with in some tunnels, and which render ventilation still 
more necessary. Tunnels may be ventilated either by natu- 
ral or artificial means. 

Natural Ventilation. — It is well known that if two rooms of 
different temperatures are put in communication with each 
other, e.g., by opening a door, a draft from the colder room will 
enter the other from the bottom, and a similar draft at the top, 
but with a contrary direction, will carry the hot air into the 
colder room, thus producing perfect ventilation, until the two 
rooms have the same temperature. Now, during the construc- 
tion of tunnels the temperature inside may be considered as 
constant, or independent of the outside atmospheric variations ; 
hence during summer and winter, there will always be a draft 
affording ventilation, owing to the difference of temperature in- 
side and outside the tunnel. In winter time the cold air out- 
side will enter at the bottom of the entrances and headings, or 
along the sides of the shafts, and the hot air will pass out near 
the top of the headings or entrances or the center of the shafts ; 
in summer the air currents will take the contrary direction. 

Natural ventilation in tunnels is improved Avhen the exca- 
vation of the heading reaches a shaft, because the interior air 
can then communicate with the exterior at two points, at dif- 
ferent levels. In such cases a force equal to the difference in 
weight between a column of air in the shaft and a similar one 
of different density at the entrance of the tunnel, will act upon 
the mass of air in the tunnel and keep it in movement, thus 
producing ventilation. (Consequently, during winter, when the 
outside air has greater weight than that inside, the air will 
come in by the headings and go out by the shaft, and in the 
summer it will enter at the shaft and pass out at the entrance. 



292 TUNNELING 

Sometimes to afford better ventilation shafts 8 or 12 in. in di- 
ameter are sunk exclusively for the purpose of changing the 
air. When the inside temperature is equal to that outside, 
as often happens during the spring and autumn, there are no 
drafts, and consequently the air in the excavation is not re- 
newed and becomes foul ; then fires are lighted under the 
shaft and a draft is artificially produced. The hot air going 
out through the shaft, as through a chimney, allows the fresh 
air to come in as in ordinary ventilation. 

When the head of the excavation is very far from the en- 
trances, or when the mountain is too high to allow excavation 
by shafts, it is quite impossible to secure good natural ventila- 
tion, especially during the spring and autumn months, and the 
engineer has to resort to some artificial means by which to 
supply fresh air to the workmen. 

Artificial Ventilation. — Artificial ventilation in tunnels may 
be obtained in two different ways, known as the vacuum and 
plenum methods. Their characteristic difference consists in 
this, that in the vacuum method the air is drawn from the in- 
side and the vacuum thus produced causes the fresh air from 
the outside to rush in, while the plenum method consists in 
forcing in the fresh air which dilutes the carbonic air produced 
inside the tunnel by workingmen and explosives. In the vac- 
uum method the pressure of the atmosphere inside the tunnel is 
always less than the pressure outside, while in the plenum 
method the pressure within is always greater than that outside. 
Ventilation is the result of this difference of pressure, as the 
tendency of the air toward equilibrium produces continuous 
drafts. Both these methods have their advantao-es and disad- 
vantages ; but in the presence of hard rock, when explosives are 
continually required, the vacuum method is considered the best, 
because the gases attracted to the exhaust pipes are expelled 
without passing through the whole length of the tunnel, thus 
avoiding the trouble that a draft of foul air will give to the 
workmen who are within the tunnel. In both these methods it 



VKNTILATI )N AND LIGHTING DUlllNG CONSTRUCTION 293 

is necessary t.) separate the fresh air from the foul one ; and this 
is done by means of pipes which will exhaust and expel the 
foul air in the vacuum method, or force to the front a current 
of fresh air when the plenum method is used. Artificial venti- 
lation may also be obtained by compressed air wdiich is set free 
after it has driven the machines, especially in tunnels excavated 
through rock, when rock drilling machines moved by com- 
pressed air are employed. 

Vacuum Method Contrivances. — The most common of the vac- 
uum appliances consists in the simple arrangement of a pipe 
leading from the head of the tunnel out through the fire of a 
furnace. The air in the pipe is rarefied by the heat of the fur- 
nace and then set free from the other end of the pipe, thus 
creating a partial vacuum in the pipe, into which the foul air of 
the head rushes, the fresh air from the entrance taking its place, 
and thus ventilating the tunnel. A similar arrangement may 
be used with shafts, and the foul air may be driven out by a 
furnace which is placed either at the top or bottom of the shaft. 
Such furnaces act the same as those commonly used for heating 
purposes in the houses, with this difference, that, instead of fresh 
air being forced in, foul air is expelled. Another simple 
arrangement for producing a vacuum is by means of a steam 
jet which is thrown into the pipe, and which helps the expul- 
sion of the air by heating it, thus producing a different density 
which originates a draft besides that mechanically originated by 
the force of the steam jet, which tends to carry out the foul air 
of the pipes. 

Foul air may also be expelled by means of exhaust fans 
which are connected with pipes near the entrance of the tunnel. 
The fan consists of a box containing a kind of a paddle wheel 
turned by steam or water power and arranged so as to revolve 
at a high si>eed. Tiie air inside the pipe is forced out by 
blades attached to the wheel, and thus the foul air of the front 
is driven away and fresh air from the entrance rushes in to take 
its place, and perfect ventilation is obtained. 



294 TUNNELING 

The best manner of expelling foul air from tunnels, accord- 
ing to the vacuum method, is by means of bell exhausted. 
This consists of two sets of bells connected by an oscillating 
beam and balancing each other. Each set consists of a movable 
bell, which covers and surrounds a fixed bell with a water joint. 
In the central part of the fixed bell there are valves which open 
upwards, and on the bottom of each movable bell there are 
valves which open from the outside. When one bell ascends, 
the valves at the bottom are closed, the air beneath is then 
rarefied, and a vacuum is produced ; the valves in the central 
part of the fixed bell filled with water are opened, and there is 
an aspiratory action from the pipe leading to the headings, and 
the foul air is thus carried away. The apparatus makes about 
ten oscillations per minute, and the dimensions of the bells 
depend upon the quantity of air to be exhausted in a minute. 
In the St. Gothard tunnel, where these bell exhausters were 
used, they exhausted 16,500 cu. ft. of air per minute. 

Plenum Method Contrivances. — Fresh air may be driven into 
tunnels to dilute the carbonic acid by two different ways, viz., 
by water blast and by fans. Water when running at a great 
velocity produces a movement in the air which may be some- 
times usefully and economically employed for ventilating 
tunnels. Water falling vertically is let run into a large 
horizontal zinc pipe having a funnel at the outer end ; into this 
the air attracted by the velocity of the water is forced. By an 
opening at the bottom the water is afterward withdrawn from 
the pipe, and there remains only the air which is pushed for- 
ward by the air which is being continually sucked in by the 
velocity of the water. 

The best and most common means of ventilation by the 
plenum method is by fans. There are numerous varieties of 
these fans in the market, but they all consist of a kind of fan 
wheel which by rapid revolution forces the fresh air into the 
pipe leading to the headings of the tunnel or to the working 
places. Instead of a large single fan, such as is used for min- 



VENTILATION AND LIGHTING DUR1N(; CONSTRUCTION 295 

ing purposes, it is better to have a number of small fans acting 
independently of each other, conveying the fresh air where it is 
needed through independent pipes. 

Compressed Air. — In the excavation of tunnels in hard rock 
a number of rock drilling machines are employed which are 
moved by compressed air at a pressure of not less than five 
atmospheres. At each stroke about 100 cu. ins. of compressed 
air is set free, and at an average of 10 strokes per minute there 
would be 5,000 cu. ins. of air at five atmospheres or 25,000 
cu. ins., or a little more than 175 cu. ft. of fresh air at normal 
pressure set free every minute by each of the machines employed. 
But the air exhausted from the drilling machine is foul. 

Regarding ventilation by compressed air, Mr. Adolph Sutro, 
in a lecture delivered to the mining students of the University 
of California, said : 

"I will note a curious fact which I have never seen explained, and which is 
worthy of close investigation by means of experiments. In the Sutro tunnel we 
found that the compressed air used for driving the machine drills, after having been 
compressed and expanded and discharged from the drills, was not wholesome to 
breathe, and the men and mules would all crowd around the end of the blower pipe 
to get fresh air. Whether the air in being compressed has parted with some of its 
oxygen or because vitiated from some other cause, I do not know, and I hope that 
this subject will at some future day be carefully examined into." 

In the December, 1901, number of the '' ^ Compressed A'h\^ "* a 
magazine especially devoted to the useful application of com- 
pressed air, is read : 

Compressed air wasted from power drills is so contaminated with oil from 
the cylinders that it cannot be taken into consideration as ventilation. It is as 
important to displace it with pure air as it is to drive out or draw off other vitiated 
air. The ventilation should be an independent supply provided by fan or blower, 
delivering by pipe at the point where miners are working. 

Quantity of Air — The quantity of air to be introduced into 
tunnels must be in proportion to the oxygen consumed by the 
men, the animals, and the explosions. It is allowed that the 
quantity of air required for breathing purpose and explosions is 
as follows : 

1 workman with lamp needs 240 cu. yds. of fresh air in 24 hours. 
1 horse " 850 " ** " " 

1 lb. gunpowder 100 " " " 

1 lb. dynamite 150 " " ** 

2070 64 



^96 TUNNELING 

In a long tunnel excavated through hard rock the number 
■of workmen all together may be assumed at 400 at each end, 
and each workman is supposed to be furnished with a lamp. 
No less than ten horses are employed, and the average quantity 
of dynamite consumed is 600 lbs. per day. From the data given 
the consumption of air by workmen and lamps would be : 
240 X 400 = 96,000 cu. yds. ; the consumption of air by horses 
would be 860 x 10 = 8,500 cu. yds. ; the consumption of air by 
dynamite would be 150 x 600 = 99,000 cu. yds. ; making a total 
consumption of air per day of 197,500 cu. yds., or about 8,000 
■cu. yds. per hour. 

To obtain good ventilation, then, it will be necessary to 
furnish every hour a quantity of fresh air amounting to not less 
than 8,0.00 cu. yds. Since, however, a large quantity of pure 
air is expelled with the foul air, it is necessary greatly to in- 
crease this quantity. 

It may be observed, in closing, that the water having its 
particles divided, as in a fog or mist, rapidly precipitates the 
gases produced by explosions. Now, when hydraulic machines 
.are used, there is a hollow ball pierced by holes that are almost 
imperceptible, from which the compressed water spreads in very 
subtile particles, and this causes the fall of the gases from 
explosions. Such a method of precipitating gases is very good, 
lout does not have the advantage of supplying new oxygen to 
leplace that consumed by the men, animals, lamps, and ex- 
plosions ; besides, it has the defect of increasing the quantity of 
water to be removed. In tunnels the pipes used either for con- 
veying the fresh air or for carrying away the foul air, are of 
iron, having a diameter of about 8 in. ; they are fixed along the 
.:side walls about 3 ft. above the inverted arch. 



VENTILATION AND LIGHTING DURING CONSTRUCTION 297 



LIGHTING. 

The object and necessity of a perfect lighting of the tunnel- 
workings during construction are so obvious that they need not 
be enlarged upon. Comparatively few tunnels require lighting 
after completion ; and these are generally tunnels for passenger 
traffic under city streets, of which the Boston Subway is a rej)- 
resentative American example. Considering the methods of 
lighting tunnels during construction, we ma}', for sake of con- 
venience, chiefly, divide the means of supplying light into (1) 
lamps and lanterns usually burning oil ; (2) coal-gas lighting; 
(3) acetylene gas lighting; and (4) electric lighting. 

Lamps and Lanterns. — Lamps and lanterns are commonly 
employed by engineers for making surveys inside the tunnel, and 
to light the instrument. For ranging in the center line, a con- 
venient form of lamp consists of an oil light inclosed in glass 
chimney covered with sheet metal, except for a slit at the front 
and back through Avhich the light shines, and on which the 
observer sights his instrument. To direct the operations of his 
rodmen the engineer usually employs a lantern, either with 
white or colored glass, much like the ordinary railway train- 
man's lantern, which he swings according to some prearranged 
code of signals. 

Lamps and lanterns are used by the Avorkmen both for sig- 
naling and for lighting the woi kings. For signaling purposes 
red lanterns are usually placed to denote the presence of unex- 
ploded blasts or other points of possible danger ; and colored or 
white lights are usually placed on the front and rear of spoil 
and material trains. For lighting purposes, two forms of lamps 
are employed, which may be somewhat crudely designated as 
lamps for individual use and lamps for general lighting. Indi- 
vidual lamps are usually of small size, and burn oil ; they may 
be carried in front of the miner's helmet, or be fixed to stand- 
ards, which can he set up close to the work being done by each 



298 TUNNELING 

man. Miners' safety lamps should be employed where there is 
danger from gas. A great variety of lamps for mining and 
tunneling purposes are on the market, for descriptions of 
which the reader is referred to the catalogues of their manu- 
facturers. 

Lamps for general lighting are always of larger size than 
lamps for individual use. A common form consists of a cyl- 
mder ten or twelve inches in diameter, provided wdth a hook or 
bail for suspension, and filled with benzine, gasolene, or other 
similar oil. Connected with this cylinder is a pipe of con- 
siderable length and small diameter through which the benzine 
or gasolene vapor runs, and burns when lighted with a brilliant 
flame. Lamps of this type burning gasolene were extensively 
employed in building the Croton Aqueduct Tunnel. Various 
patented forms of lamps for burning coal-oil products are on 
the market, for descriptions of which the manufacturers' cata- 
logues may be consulted. 

Coal-gas Lighting. — A common method of lighting tunnel 
workings is by piping coal-gas into, the headings and drifts from 
some nearby permanent gas plant, or from a special gas works 
constructed especially for the work. Gas lighting has the great 
advantage over lamps and lanterns of giving a light which is 
more brilliant and steady. Its great objection is the danger of 
explosion caused by leaks in the pipes, by breaks caused by 
flying fragments of rock, and by the carelessness of wot-kmen 
who neglect to turn off completely the burners when they ex- 
tinguish the lights. In nearly every tunnel where gas has been 
used for lighting, the records of the work show the occurrence 
of accidents which have sometimes been very serious, partic- 
ularly when fire has been communicated to the tunnel tim- 
bering. 

Acetylene Gas Lighting. — The comparatively recent devel- 
opment of acetylene gas manufactured from carbide of calcium 
has given little opportunity for its use in tunnel lighting, and 
the only instance of its use in the United States, so far as the 



VKNTILATIOX AND LIGHTING DURING CONSTRUCTION 299 

author knows, is the water-works tunnel conduit for the city of 
Washington, D.C. Col. A. M. Miller, U.S. Engineer Corps, 
who is in charge of this work, describes the method adopted in 
his annual report for 1899 as follows : — 

''It had been the practice to do all work underground by the light of 
miners' lamps and torches. This means of illumination is very poor for me- 
chanical work. The fumes and smoke from blasting, added to the smoke from 
torches and lamps, render the atmosphere underground, especially when the 
barometer conditions were unfavorable to ventilation, very offensive and dis- 
comforting to the workmen. An investigation of the subject of lighting the 
tunnel by other means, more especially at the locality where the mechanics 
were at work, — brick and stone masons, and the workmen on the iron lining, 
— resulted in the selection of acetylene gas as the most available and economi- 
cal in this special emergency. Accordingly, an acetylene gas plant for 300 
burners was erected at Champlain-Avenue shaft, and one for 60 lights at Foun- 
dry Branch. The engine-houses at the shafts, the head-houses, and localities 
in the tunnel, when required, are lighted by these plants, 

" Gas pipes were carried down the Champlain-Avenue shaft and along the 
tunnel both in an easterly and westerly direction, with cocks for burners at 
proper intervals every 30 feet ; and this system sufficed for illumination from 
Rock Creek to Harvard University, a distance of over two miles. The plant 
erected at Foundry Branch was in like manner utilized for the illumination 
from that point in both directions. 

" By connecting with the stopcocks by means of a rubber hose, a movable 
light, chandelier, or 'Christmas-tree' of any required number of burners is 
u.sed, thus concentrating the light in the immediate vicinity of the work, and 
also enabling the illumination to be carried into the cavities or 'crow- nests,' so 
called, behind the defective old lining. 

•'This method of illuminating has proved very satisfactory and quite eco- 
nomical. It is especially valuable as enabling good work to be done, and 
facilitating a thorough inspection of the same." 

Electric Lighting. — By far the most perfect, and at present 
the most commonly employed means for lighting tunnel work- 
ings, is electricity. The light furnished by electric lamps is 
steady and bnlliant, and does not consume oxygen or give off 
offensive gases. The wires are easily removed and extended, 
and the lamps are easily put in place and removed. About the 
only objection to the method is the fragility of the lamps, which 
are easily broken by the flying stones and the concussion pro- 
duced by blasting. 



300 TUNNELING 



CHAPTER XXV. 

THE COST OF TUNNEL EXCAVATION AND 
THE TIME REQUIRED FOR THE WORK. 



Cost. — The cost of a tunnel will depend upon the cost of 
the two principal operations required in its construction, viz., 
the excavation of the cross section and the lining of the exca- 
vation with masonry, metal, or timber. These two operations 
may in turn be subdivided, in respect to expense, into cost of 
labor and cost of materials. It is a comparatively simple mat- 
ter to calculate the cost of the building materials required to 
construct a tunnel ; but it is very difficult to estimate with 
accuracy what the cost of labor will be. The reason for this is 
that it is impossible to foresee exactly what the conditions will 
be ; the character of the material may change greatly as the 
work proceeds, increasing or decreasing the cost of excavation ; 
water may be encountered in quantities which will materially 
increase the difficulties of the work, etc. Nevertheless, while 
accurate preliminary estimates of cost are not practicable, it is 
always desirable to attempt to obtain some idea of the probable 
expense of the work before beginning it, and the more usual 
means of getting at this point will be discussed here. 

Two methods of estimating the cost of tunnel work are em- 
ployed. The first is to calculate the probable expense of the 
various items of work, based upon the available data, per unit 
of length, and then add to this a margin of at least 10% to allow 
for contingencies ; the second is to apply to the new work the 
unit cost of some previous tunnel built under substantially the 
same conditions. In the first method it is usual to consider 
the strutting and hauling as constituting a part of the work of 



COST OF EXCAVATION AND TIME REQUIRED 801 

excavation. To estimate the cost of excavation involves the 
consideration of three general items, viz., the excavation proper, 
the strutting of the walls of the excavation, and the hauling of 
the excavated materials and the materials of construction. 

The cost of excavating the preliminary headings or drifts is 
greater per unit of material removed than that of excavating 
the enlargement of the section. The cost of bottom drifts is 
also always greater than that of top headings, the material pene- 
trated remaining the same. ]\lr. Rziha gives the comparative 
unit costs of excavating drifts, headings, and enlargement of 
the profile as f ollow^s : — 

Bottom drifts $9.20 per cu. yd. 

Top headings 4.80 " " " 

Enlargement of profile 2.84 " " '♦ 

The cost of hauling increases with the length of the tunnel. 
This fact and amount of this increase are indicated by the 
following actual prices for the Arlberg tunnel : — 

Top heading 86.76 per en. yd., increasing 37 cts. per mile 

Bottom drift 7.40 " " " " 26 " " " 

Enlargement of profile . . . 2.70 " " " " 10 " " " 

In all the prices given above, the cost of strutting and haul- 
ing is included in the cost of excavation. 

The cost of excavation is not always the same for the same 
character of materials in different tunnels. The following 
fio-nres show the prices paid for the excavation of calcareous 
rock in four different German tunnels: — 

Berliner Nordhausen Wetzler R.R .'5I.24 per cu. yd. 

Ofen 1.30 " " " 

Stafflach 2.76 " " " 

Gries 1.02 " " " 

The method of tunneling has little influence upon the cost 
of the w^ork, as sho\vn by the following figures from tunnels 
excavated through calcareous rock by different methods: — 



302 TUNNELING 

Ofen tunnel Austrian mef.od 193.19 per lin. ft. 

Dorremberg tunnel Belgian method 86.08 "" " 

Stafflach tunnel English method 91.69 " " " 

The Martha and Merten tunnels, excavated through soft 
ground by the Austrian and German methods respectively, 
cost 187.95 and $87.55 per lin. ft. respectively. In the exca- 
vation of the various sections of the tunnel for the new Croton 
Aqueduct in America, the following prices were paid: — 

Excavation of heading $8 to 110.00 per cu. yd. 

Tunnel in soft ground 8 to 9.00' " " " 

Tunnel in rock 7 to 8.50 " " " 

Brick masonry 10.00 " " " 

Timber in place $40 per M. ft. B, M. 

It is the practice in America to include the work of hauling 
under excavation, but not to include the strutting, which is 
paid for separately. In some cases only the market price of 
the timber is paid for separately, the cost of setting up being 
included in the price of excavation. The writer prefers the 
European practice of including the total cost of timbering 
under excavation, since the two operations are so closely con- 
nected, and since the contractor employs the same timber over 
and over again. Knowing the dimensions of the several mem- 
bers of the strutting, it is a simple, although somewhat tedious, 
process to calculate the total quantity required. An idea of 
the quantity of timber required for strutting in soft ground 
may be had from the data given on page 50. The quantity 
will decrease as the cohesion of the material penetrated in- 
creases, until it becomes so small in hard rock-tunnels as to cut 
very little figure in the total cost. 

The cost of hoisting excavated materials through shafts 
depends upon the depth from which it is hoisted, and upon the 
character of hoisting apparatus employed. The following table, 
showing the cost of hoisting for different lifts and by different 
methods, is given by Rziha, the cost being in francs per cubic 
meter : — 



COST OF KXrAVATIOX AND TIME REQUIRED 



303 



Heicht I>' 


Windlass. 


Horse Gins. 


Steam Hoists. 








Metres. 




One Horse. 


Two Horses. 






Francs per Cu.M. 


Francs per Cu. M. 


Francs per Cu.M. 


Francs perCu. ^Nl. 


15 


0.172 


0.077 


0.002 


0.035 


30 


0.212 


0.087 


0.070 


0.045 


45 


0.257 


0.100 


0.080 


0.050 


CO 


0.305 


0.112 


0.092 


0.082 


90 


0.410 


0.152 


0.110 


0.087 


120 


0.585 


0.195 


0.135 


0.0i)2 


150 


0.722 


0.240 


0.157 


0.112 



;Mr. Sejourne, a French engineer, who has been connected 
with the construction of numerous tunnels by the Belgian 
method where he was in position to secure comparative figures, 
has given the following rules for calculating the cost of 
tunnels. Assuming A to represent the cost of excavating a 
cu. yd. in the open air, the cost of excavating the same 
quantity underground in driving headings will be from 9 J. to 
11 ^, and in enlarging the profile it will be about 5 A. The 
cost of constructing single-track tunnels varies with the thick- 
ness of the lining, and may be calculated by the following 
formulas : 

Without lining, C — 5.5 A. 

With roof arch only, C = 6.4 -f 6.4 A. 

With lining 18 in. thick, C = 9.4 + 7 A. 

With linincr 2 ft. thick, C = 11 + 8 A 



In these formulas C is the cost per cu. yd. of excavation, 
including the masonry. For double-track tunnels the amounts 
given l)y the above fornuilas may be used by reducing them 
about Ti % or 8 9;. 

The second metliod of estimating tlie cost of tunnel work 
consists in assuming as a unit the unit cost of tunnels pre- 
viously excavated under simihir conditions. Mr. La Dame 
gives the following nnit prices for a number of tunnels driven 
through different materials : 



304 



TUNNELING 



Nature of Soil. 




EXCAV. PER 


Cost per 


Max. and Mm. 


P 


Cu. Yd. 


L.l:s. Ft. 


PER LlN. Ft. 


Granite-gneiss . . 


oQ 


$3.07 @$3.85 


1100. 


$61.46 @ $190.40 


Schist 


39 


1.38 @ 1.53 


75.42 


43.11 @ 70.68 


Triassic 


3 


. 


90.85 


84.75 @ 93.33 


Jurassic .... 


69 


1.23 @ 1.38 


77.86 


35.24 @ 157.2 


Cretaceous .... 


34 


0.61 @ 0.77 


59.60 


27.37 @ 92.25 


Tertiary and modern 


39 


0.33 @ 0.61 


105.80 


51.52 @ 188.36 



In the following table is given a list of tunnels excavated 
through different soils, from the most compact to very loose 

DOUBLE-TRACK TUNNELS. 



Name of Tunnels. 


Quality of Soil. 


Cost per 
Lin. Ft. 


Method of 
Tunneling. 


Mt. Cenis 

St. Gothard 

Stammerich 

Stalle 

Bothenfels 

Dorremberg .... 
Stafflach ...... 

Ofen 

Wartha 

Mertin 

Scloss Matrei .... 
Trietbitte 


Granitic, 

Granitic, 

Broken schist. 
Dolomite, 
Calcareous, 
Calcareous, 
Calcareous, 
Grev^ack, 
Grewack, 
Clay schist. 

Clay and sand, 


$273.73 

193.63 

157.90 

290.58 

115.64 

86.08 

91.69 

93.19 

87.95 

87.55 

94.25 

229.0 

69.50 

178 

182.31 


Drift. 

Heading. 

English. 

Austrian. 

English. 

Belgian. 

English. 

Austrian. 

Austrian. 

German. 

English. 

German. 

Wide heading. 


Canaan 

Church-Hill .... 
Bergen No. 1 . . . . 


Clay-slate, 
Clay with shells. 
Trap rock. 



SINGLE-TRACK TUNNELS. 



Name of Tunnels. 


Quality of Soil. 


Cost per 
Lin. Ft. 


Method of 
Tunneling. 


Mt. Cenis 

Stalletti 

Marein 

Welsberg 

Sancina 

Starre 

Cristina 

Burk 

Brafford Ridge . . . . 

Dunbeithe 

Fergusson 

Port Henry 

Points 


Gneiss, 
Granite and quartz, 

Clay schist. 

Gravel, 
Clay of 1st variety. 
Clay of 2d variety. 
Clay of 3d variety. 

Limestone, 
Sandstone, 
Limestone, 
Granite, 


$82.27 
62.75 
64.36 
165 07 
129.40 
191.61 
307.42 
83.90 
85.33 
70.47 
37.46* 
SO.OOt 
72.00* 


Heading. 

Austrian. 

English. 

Austrian. 

Belgian. 

Belgian. 

Italian. 
Wide heading. 
Wide heading. 
Wide heading. 
Wide heading. 
Wide heading. 
Wide heading. 



* Are unlined. 



t Lined with timber. 



COST OF EXCAVATION AND TIME llEQUIRED 305 

materials, and driven according to the various methods which 
have been illustrated. 

The Habas tunnel through quicksand, between Dax and 
Ramoux, France, cost ?S^118.50 per lin. ft. The cost of 
the Boston subway was i>3-i:2.40 per lin. ft. The Severn 
and Mersey tunnels, constructed through rock under water, 
cost respectively $208.33 and |263 per lin. ft. The First 
Thames Tunnel, driven by Brunei's shield, cost ^$^1 661.66 per 
lin. ft. The Hudson Kiver and St. Clair River tunnels, exca- 
vated through soft ground by means of shields and compressed 
air, cost respectively #305 and 1315 per lin. ft. The Black- 
wall double-track tunnel under the River Thames, which is 
the largest tunnel ever built by the shield system, cost -1400 
per lin. ft. 

In making estimates of the cost of projected tunnel work 
based on the cost of tunnels pieviously constructed through 
similar materials, it is important to keep in mind the date and 
location of the work used as the basis for calculations. For 
example, a tunnel excavated in Italy, where labor is very cheap, 
will cost less than one excavated in America, Avhere labor is 
dear, all other conditions being the same. Other reasons for 
variation in cost due to difference of date and location of con- 
struction will suggest themselves, and should be taken into full 
consideration in estimating the cost of the new work. 

Time. — The time required to excavate a tunnel depends 
upon the character of the material pene tinted and upon the 
method of work adopted. Tunnels driven through soft ground 
by hand require al)ont tlie same time to construct as tunnels 
driven through hard rock by the aid of machinery. Tunnels 
can be driven through hard rock at about as great a speed as 
through soft or fissured rock, chiefly because the work of 
blasting is more efficient in hard rock, nnd because no time 
is required in timbeiiiifr. The following table shows the 
average rate of prorrrcss in different parts of the tunnel excava- 
tion tlirongh both liard and soft materials in feet per month: — 



306 



TUNKELING 



Quality of Soil. 


Heading. 


Excavation of Shafts. 


Enlarge- 
ment of 
Profile. 


By hand. 


By machine. 


By hand. 


By machine. 


By hand. 


Very loose soil . 
Loose soil . . 
Soft rock . . . 
Hard rock . . 
Very hard rock. 


16.7 -26.8 

33.4 -100 

66.8 

50 -66.8 

33.4 


233.8-334 
233.8-334 
233.8-334 


6.6-16.7 
16.7-33.4 
33.4-66.8 
33.4-50 
16.7-33.4 


66.8-132.6 
66.8-132.6 
66.8-132.6 


6.6-16.7 
16.7-33.4 
33.4-50 
66.8-100 
66.8-100 



The following tables showing the average rate of progress 
have been compiled from the actual records made in the 
tunnels named : 



Name of Tunnel. 


Dimensions 
IN Feet. 


m 
m 


Character of 
Material. 


Observations. 


Excavation of headings 










by hand: 










Mount Cenis . . 


10X10 


65.8 


Schist, 


Bottom drift. 


Sutro 


6.7X5.7 


70.14 


Quartzose, 


. 


St. Gothard . . . 


8.4x8.7 


70.14 


Granite, 


Top heading. 


Excavation of headings 










by machine: 










Mount Cenis . . 


10 X 10 


188.7 


Calcareous schist, 


Bottom drift. 


Sutro ... 


8.15X10 


227.45 


Quartzose, 




St. Gothard 




8.4x8.7 


339.45 


Granite, 


Top heading. 


Trari . . 




8 X 9.35 


167 


Gneiss, 


Top heading. 


Arlberg . 




8.35 X 9.35 


474.2 


Mica schist, 


Bottom drift. 


Palisades . 




16X7 


160 


Trap rock. 


Top heading. 


Busk . . 




15x7 


126 


Granite, 


Top heading. 


Cascade . 




16X8 


180 


Basaltic rock. 


Top heading. 


Franklin .... 


15X7 


240 




Top heading. 



The following table shows the monthly progress of com- 
pleted tunnel in feet exc? vated through rock : 



Name of Tunnel. 


Progress 
IN Feet. 


Material. 


Method. 


Cascade 

Palisades 

Busk 

Tennessee Pass 


207 
186 
190 
169.5 


Basalt, 
Trap rock. 
Granite, 
Granite, 


Top heading. 
Top heading. 
Top heading. 
Top heading. 



COST OF EXCAVATING AND TIME REQUIRED 



307 



The average monthly progress in feet of excavating tunnels 
through treacherous ground may be quite generally assumed 
to be for: (1) clay of the first variety from 43.4 ft. to 60 ft. ; 
for clay of the second variety from 33.4 ft. to 43.4 ft. ; for clay 
of the third variety from 23.3 ft. to 33.4 ft., and for quicksand 
from 30 ft. to 50 ft. The monthly progress in feet made in 
sinking the shafts of the Hoosac and Musconetcong tunnels in 
America was as follows : — 



Name of Tcnnkl. 


DiMEX.SIONS 

IN Feet. 


Depth 
IN Feet. 


Progress 
IN Feet. 


Character 
OP Material. 


Hoosac: 

East shaft 

West shaft 

Musconetcong: 

Vertical shaft .... 

Inclined shaft .... 


15.4 X 27.7 
8X16 

8.35 X 16.7 
8.35 X 26 


1035 
267 

113.5 
304. 


21.7 

16.7 

100 
32 


Mica schist. 
Gneiss. 

Loose rock. 
Loose rock. 



The average monthly progress of sinking shafts in treach- 
erous soils may be assumed to be as follows : clay of first 
variety, 50 ft. to 75 ft. ; clay of second variety, 36.75 to 50 ft; 
clay of third variety, 23.4 ft. to 36.75 ft. ; quicksand, 16.7 ft. 
to 33.4 ft. 



INDEX 



PAGE 

Air Compressors, Description of . , 81 

Accidents in Tnnnels 1'12, 154 

After Construction 273 

Chattanooga Tunnel 276 

During Construction 266 

Giovi Tunnel 274 

Repairing of 269 

Baltimore Belt I.ine Tunnel, Gen- 
eral Description 1~yO 

Boston Sul)\vay, General Description . 187 
Busk Tunnel, General Description . .119 

Cascade Tunnel 129 

Center Line, Determination ol . . . 9 

Curvilinear Tunnels .... 13 

Rectilinear Tunnels .... 9 

Siniplon Tunnel 276 

Chattanooga Tunnel, Accident in . . 276 

Cross-Section : 

Boston Subway 187 

Dimensions of 17 

Form of 15 

New York Rapid Transit Railway 

Tunnel 195 

Croton Aqueduct Tunnel 126 

Culverts 75 

Drills: 

Hand 20 

Power 21,84 

Percussion 21 

Mountings for . . .22 

Rotary 22, 102 

Kast River Gas Tunn«'l, General 

Description 208 

Entrances 77 

Kxcavation : 

Austrian method 162 

Advantages and r>isadvantages 166 

Baltimore Belt Line Tunnf 1 . . . 151 

Belgian Method 1.^5 

Advantages and Disadvantages 142 

Boston Subway 188 



Excavation : (continued) page 

Busk Tunnel 120 

Cost of .300 

Division of Section 32 

English Method 156 

Advantages and Disadvantages 161 

Enlargement of Profile 34 

German Method 145 

Advantages and Disadvantages 149 

Heading.s 33 

Italian method 167 

Advantages and Disadvantages 173 

Mont Cenis Tunnel 89 

Rock, Methods of 85 

Quicksand Method ..... 174 

Simplon Tunnel 100 

Tunnels, open-cut : 

Parallel Longitudinal Trenches 181 

Single Longitudinal Trenches . 180 

Transverse Trenches .... 182 
Excavators : 

Machines for Earth 19 

Machines for Rock 20 

Explosives : 

Dynamite 26 

Gunpowder 25 

Nature of . . 29 

Nitroglycerine . ..... 26 

Storage of 27 

Use in Blasting 25 



Fuses 



27 



Geological Surveys : 

Method and Purp se of . . . . 3 

Simplon Tunnel 96 

Graveholz Tunnel 129 



Haulin;; : 

Belgian Method . . 
By Way of Entrances 
liy Way of Shafts . 
Definition .... 
p]nglish Method . . 
German Metliod 
Italian Method . . 



140 

55 
58 
55 
161 
148 
170 



309 



310 



INDEX 



Hsiniing : (continued) page 

Mont Cenis Tunnel 92 

Saint Gothard Tunnel 119 

Simplon Tunnel 102 

Hoisting Machinery 58 

Hoosac Tunnel 124 

Jacks c 263 

Liigliting : 

Acetylene Gas 298 

Coal Gas , . , , . 298 

Electricity 299 

Lamps and Lantei'ns 297 

Linings : 

Iron 69, 215 

Iron and Masonry : ...... 70 

New York liapid Transit Ry. 

Tunnel . 196 

Masonry : 70 

Accidents and Repairs to . . 142 

Austrian Method 165 

Baltimore Belt Line Tunnel . 152 

Belgian Method 137 

Boston Subway 190 

Centers for Arches 64 

English Method ..... 159 

German Method 148 

Ground Molds for 62 

Invert 73 

Italian Method 169 

Leading Frames for 63 

Mont Cenis Tunnel 92 

Quicksand Method ..... 176 

Roof Arch 72 

Saint Gothard Tunnel .... 117 

Side Walls 72 

Thickness of 74, 78 

Purpose of 68 

Timber 68 

Machinery, Hoisting 58 

Materials, Character of : 3 

Baltimore Belt Line Tunnel . . . 150 

Boston Subway 187 

Busk Tunnel 119 

Mont Cenis Tunnel 88 

New York Rapid Transit Tunnel . 194 

Saint Gothard Tunnel 116 

Milwaukee Water-works Tunnel . . . 230 
Mont Cenis Tunnel, General Descrip- 
tion 87 

New York Rapid Transit Railway, 

Tunnel, General Description . , . 192 

Niagara Falls Power Tunnel .... 128 

Niches 76 



PAGF, 

Open-cut, Choice Between a Tunnel 

and 1 

Palisades Tvmnel 125 

Power-Plants : 

Air compressors ....... 81 

Busk Tunnel 121 

Canals and Pipe Lines ..... 81 

Cascade Tunnel 129 

Croton Aqueduct Tunnel .... 126 

General Description ..... 79 

Graveholz Tunnel 129 

Hoosac Tunnel 124 

Mont Cenis Tunnel 90 

Niagara Falls Power Tunnel . . . 128 

Palisades Tunnel 125 

Receivers 84 

Reservoirs 81 

Saint Clair River Tunnel .... 130 

Saint Gothard Tunnel 117 

Simplon Tunnel = 108 

Sonnstein Tunnel 130 

Steam 80 

Strickler Tunnel 127 

Turbines 81 

Saint Clair River Tunnel .... 130 
Saint Gothard Tunnel, General De- 
scription 114 

Severn Tunnel, General Description . 204 

Shafts, Description of 36 

Shield Construction : 

Cellular Division 258 

Diaphragm 259 

Front End 257 

General Form ........ 255 

Jacks 263 

Rear End 261 

Shell 256 

Shields : 

Barlow's 246 

Blackwall Tunnel ......'. 252 

Boston Subway 255 

Broadway Pneumatic Railway Tun- 
nel ... o . o ..... 249 
City and South London Railway 

Tunnel 250 

Clichy Sewer Tunnel 253 

East River Gas Tunnel 218 

First Thames Tunnel .245 

North and South Woolwich Subway 249 

Saint Clair River Tunnel .... 251 

Simplon Tunnel, General Description . 94 

Sonnstein Tunnel 130 

Strata, Inclination of 6 

Strickler Tunnel 127 



INDEX 



311 



Strutting : page 

Austrian Method 163 

Baltinu>re Belt Line Tunnel ... 151 

Belgian .^letbod . : 136 

English Method ..... 157 

German Method . 146 

Iron : 

Full Section . . .... 52 

Headings 52 

Shafts 53 

Italian Methoil 168 

Mont Cenis Tunnel 91 

Quicksand ^lethod 175 

Saint Gothard Tunnel .... 116 
Timber : 

Dimensions » . 50 

Face of Excavation 47 

Full Section 47 

Headings , . 44 

Quantity , 50 

Shafts . . < ., 48 

Tamping . . . 29 

Timbering (See Strutting). 
Tunnels : 

Classification of ..<,.... . 38 

Hard Rock ........ 39 

Loose Soil c . 39 

Open-cut 40 

Quicksand 40 

Submarine c 41 

Historical Development .... ix 

Open-cut, General Description . . 180 

Relining of : 

Boulder Tunnel ,280 

Little Tom Tunnel 286 

MuUan Tunnel ....>.. i84 



Tunnels : (continued) page 

Soft Ground : 

Austrian Method ..... 162 

English Method .156 

General Discussion ..... 133 

German Method 145 

Italian Method 167 

Pilot Method 177 

Quicksand Method 173 

Submarine : 

East River Gas Tunnel ... 208 
General Description . , . . 'JOl 
Milwaukee Water-Works Tun- 
nel 230 

Severn Tunnel ....... 204 

Shield System, History of De- 
velopment 242 

"Van Buren Street Tunnel, 

Chicago 225 

Surface . 183 

Under City Streets : 

Boston Subway 186 

General Discussion . . . o . 184 
New York Rapid Transit Rail- 
way Tunnel . . c . . . 192 

Van Buren Street Tunnel, Chicago 225 

Ventilation : 

Artificial 292 

Boston Subway c . • 191 

Mont Cenis Tunnel ...... 92 

Natural c . 291 

Plenum Method . 294 

Simplon Tunnel Ill 

Vacuum Method ....... 293 

Water, Presence in Tunnels . . .7 



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** Compressed Air." 

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A practical treatise on this subject, containing valuable in- 
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SCIENTIFIC PUBLICATIONS. 23 



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William Cain, C.E. 

No. 49. ON THE MOTION OF A SOLID IN A FLUID. By 

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Fifth edition. 

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A. de Varona. 

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ALGEBRAIC NUMBERS. By Prof. W. Cain. 

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No. 7«;. RECENT PROGRESS IN DYNAMO-ELECTRIC MA 

CHINES. /Being a Supph 
Prof. Sylvanus P. Thompson. 



CHINES. Being a Supplement to Dynamo-Electric Machinery. By 
- - ~ - - ^j 



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By Lieut. James S. Pettit, U.S.A. 

No. 77. STADIA SURVEYING. The Theory ot Stadia Measurements. 

By Arthur Winslow. 

No. 78. THE STEAM-ENGINE INDICATOR, AND ITS USE 

By W. B. Le Van. 

No. 79. THE FIGURE OF THE EARTH. By Frank C. RobertS,C.E. 

No. 80. HEALTH V FOUNDATIONS FOR HOUSES. By Gle»ii 

brown 



SC/EATE SERIES. 



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ACCURACY, DELIVKRV, ETC. Distinctive features of the Worth- 
ington, Kennedy, Siemens, and Hesse meters. By Ross E. Browne. 

No. 82. THE PRESERVATION OF TIMBER BY THE USE 

OF ANTISEPTICS. ]5y Sauuiel Dagstcr Boulton, C.E. 

No. 83. MECHANICAL INTEGRATORS. r>y Prof Henry S. H. 
Shaw, C.E. 

No. 84. FLOW OF WATER IN OPEN CHANNELS, PIPES, 
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No. 85 THE LUMINIFEROUS ^THER. ByProf.de Volson Wood. 

No. 86. HAND-BOOK OF MINERALOGY; DETERMINATION 
AND DESCRIPTION OF MINERALS FOUND IN THE UNITED 
STATES. By Prof. J. C. Foye. 

No. 87. TREATISE ON THE THEORY OF THE CON- 
STRUCTION OF HELICOIDAL OBLIQUE ARCHES. By John 
L. Culley, C.E. 

No. 88. BEAMS AND GIRDERS. Practical Formulas for their Re- 
sistance. By P. H. Philbrick. 

No. 89. MODERN GUN-COTTON: ITS MANUFACTURE, 
PROPERTIES, AND ANALYSIS. By Lieut. John P. Wisser, U.S.A. 

No. 90. ROTARY MOTION, AS APPLIED TO THE GYRO- 
SCOPE. By Gen. J. G. Barnard. 

No. 91. LEVELING: BAROMETRIC, TRIGONOMETRIC, AND 
SPIRIT. By Prof. I. O. Baker. 

No. 92. PETROLEUM : ITS PRODUCTION AND USE. By 

Boverton Redwood, F.I.C., F.C.S. 

No. 93. RECENT PRACTICE IN THE SANITARY DRAIN- 
AGE OF BUILDINGS. With Memoranda on the Cost of Plumbing 
Work. Second edition, revised. By William Paul Gerhard, C. E. 

No. 94. THE TREATM'ENT OF SEWAGE. By Dr. C. Meymott 
Tidy. 

No. 95- PLATE GIRDER CONSTRUCTION. By Isami Hiroi, C.E. 
Second edition, revised and enlarged. Plates and Illustrations. 

No. 96. ALTERNATE CURRENT MACHINERY. By Gisbert 
Kapp, Assoc. M. Inst., C.E. 

No. 97. THE DISPOSAL OF HOUSEHOLD WASTE. By W, 
Paul Gerhard, Sanitary Engineer. 

No. 98. PRACTICAL DYNAMO-BUILDING FOR AMATEURS. 
HOW TO WIND FOR ANY OUTPUT. By Frederick Walker. 
Fully illustrated. 

110.99. TRIPLE-EXPANSION ENGINES AND ENGINE 
TRIALS. By Prof. Osborne Reynolds. Edited, with notes, etc., by 
F. E. Idell. M. E. 



JUL 2 



JUL 



nA lOno 



29 1902 

SCIENCE SERIES. 



No. 100. HOW TO BECOME AN ENGINEER ; OR, THE 
THEORETICAL AND PRACTICAL TRAINING NECESSARY IN 
FITTING FOR THE DUTIES OF THE CIVIL ENGINEER. The 
Opinions of Eminent Authorities, and the Course of Study in the 
Technical Schools. By Geo. W. Plympton, Am. Soc. C.E. 

No. loi. THE SEXTANT AND OTHER REFLECTING 

MATHEMATICAL INSTRUMENTS. With Practical Suggestions 
and Wrinkles on iheir Errors, Adjustments, and Use. With thirty- 
three illustrations. By F. R. Braiiiard, U.S.N. 

No. 102. THE GALVANIC CIRCUIT INVESTIGATED 

MATHEMATICALLY. By Dr. G. S. Ohm, Berlin, 1827. Translated 
by William Francis. W..I1 Preface and Notes by the Editor, Thomas 
D. Lockwood, M.I.E.E. 

No. 103. THE MICROSCOPICAL EXAMINATION OF POTA- 
BLE WATER. With Diagrams. By Geo. W. Rafter. 

■^o. 104. VAN NOSTRAND'S TABLE-BOOK FOR CIVIL AND 
MECHANICAL ENGINEERS. Compiled by Geo. W. Plympton, C.E. 

No. 105. DETERMINANTS, AN INTRODUCTION TO THE 

STUDY OF. With examples. By Prof. G. A. Miller. 

No. 106. TRANSMISSION BY AIR-POWER. Illustrated. By 
Prof. A. B. W. Kennedy and W. C. Unwin. 

No. 107. A GRAPHICAL METHOD FOR SWING-BRIDGES. 

A Rational and Easy Graphical Analysis of the Stresses in Ordinary 
Swing-Bridges. With an Introduction on the General Theory of Graphi- 
cal Statics. 4 Plates. By Benjamin F. LaRue, C.E. 

No. 108. A FRENCH METHOD FOR OBTAINING SLIDE- 
VALVE DIAGRAMS. 8 Folding Plates. By Lloyd Bankson, B.S., 
Assist. Naval Constructor, U.S.N. 

No. 109. THE MEASUREMENT OF ELECTRIC CURRENTS. 

Electrical Measuring Instrujvtents. By Jas. Swinburne. Meters 
FOR Electrical Energy. By C. H. Wordingham. Edited by 
T. Commerford Martin. Illustrated. 

No. no. TRANSITION CURVES. A Field Book for Engineers, 
containing Rules and Tables for laying out Transition Curves. By 
Walter G. Fox. 

No. III. GAS-LIGHTING AND GAS-FITTING, including Specifica- 
tions and Rules for Gas Piping, Notes on the Advantages of Gas for 
Cooking and Heating, and useful Hints to Gas Consumers. Second 
edition, rewritten and enlarged. .By Wm. Paul Gerhard. 

No. 112. A PRIMER ON THE CALCULUS. By E. Sherman 
Gould, C.E. 

No. 113. PHYSICAL PROBLEMS AND THEIR SOLUTION. 

By A. Bourgougnon, formerly Assistant at Bellevue Hospital. 

No. 114. MANUAL OF THE SLIDE RULE. By F. A. Halsey of 
the American Machinist. 




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